Interactions on record — worth a quick check against your medications. Based on 3 of 4 ingredients. Check your meds →
Dietary supplement

Fiber Capsules Ingredients & Drug Interactions

by Kirkland Signature

Fiber Capsules label
See full label image
Capsule Category: Fiber And Other Nutrients
Most serious interaction: Moderate
Clinical methodology reviewed by · Updated Sep 9, 2026 · Label data from the NIH DSLD
What is this page for?

First and foremost: checking Fiber Capsules against your medications. The heart of this page is the interaction checker and the full interaction report — how this product’s ingredients may interact with prescription and over-the-counter medicines you may be taking.

Around that, we add a pharmacist’s high-level view of the product as a whole — what’s inside, the evidence for its stated use, how transparent the label is, and what safety data exists — so you can see the full picture in one place. It’s educational information from our licensed clinical databases and the clinical staff at HelloPharmacist — not medical advice — and we don’t sell or endorse products. Our editorial policy

The interaction bottom line Most serious interaction: Moderate

Fiber Capsules is a dietary supplement by Kirkland Signature with 4 active ingredients. Its ingredients are commonly taken for constipation, diarrhea, high cholesterol.Based on those ingredients, 2,038 medications have a known interaction with it, the most serious rated moderate. The ingredients most likely to interact are Dietary Fiber, Iron, Potassium. Use the checker below to test your specific medication, or read the full HelloPharmacist Interaction Report.

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Check Your Meds Against Fiber Capsules by Kirkland Signature

Ask about any prescription or over-the-counter medication and we check it for interactions with Fiber Capsules by Kirkland Signature — and tell you which ingredient is responsible.

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HelloPharmacist Scorecard of Fiber Capsules by Kirkland Signature

Our pharmacy team’s full take, with four database checks built into the cards below — a summary of what is known, not a grade of the product itself.

From our pharmacy team — supplement deep dive

Generated from HelloPharmacist’s pharmacist-curated databases and editorial rules · Last generated Jul 30, 2026

What’s inside

Full disclosure
Ingredient Transparency · database check
Full

Every active ingredient lists its own amount on the label.

Why this rating?
  • The label discloses an exact amount for 3 of its 3 active ingredients.
  • No proprietary blends here — you can verify the dose of every single component.

This capsule contains 3 active ingredients: potassium, iron, and soluble fiber. Potassium is a mineral that helps regulate heart rhythm and blood pressure.

Iron supports red blood cell formation and oxygen transport, especially important if you have iron deficiency anemia. Soluble fiber absorbs water in your digestive tract and helps soften stool.

The capsule also contains inactive ingredients — gelatin, psyllium, polysorbate 80, caramel color, acacia, and black iron oxide — that serve as the capsule shell and binding material.

Does it work?

Strong evidence
Evidence for Intended Use · database check
By FDA rules, dietary supplements can’t claim to treat, cure, or prevent disease — so labels speak in careful marketing language. We discern each product’s intended use from its name, label claims, and label statements, then grade the clinical evidence for that use. How these ratings are computed
Strong

Clinical evidence supports at least one of this product's ingredients for its stated purpose.

Why this rating?
  • The label markets this product for: digestive health and heart health support.
  • We looked for evidence on: Constipation, Diarrhea, Coronary heart disease (CHD), Diabetes, Iron deficiency anemia, cholesterol management — and 1 related terms.
  • The strongest evidence on file: Black Psyllium is rated "Effective" for Constipation (Natural Medicines).
  • Also on file: Iron is rated "Effective" for Iron deficiency anemia.
  • Also on file: Black Psyllium is rated "Likely Effective" for Coronary heart disease (CHD).

Iron is established as effective for iron deficiency anemia, anemia of chronic disease, and pregnancy-related iron deficiency, and is possibly effective for heart failure. The facts we hold don't include an effectiveness rating for potassium or soluble fiber, so we can't tell you what the evidence shows for those roles in this product.

The evidence, ingredient by ingredient Black Psyllium Potassium Iron

How safe is it?

Well-documented data
Safety Information · database check
Well characterized

Adverse-effect, pregnancy, and general safety data are on file for most of these ingredients.

Why this rating?
  • We hold adverse-effect (side-effect) data for 3 of the 3 matched ingredients.
  • Pregnancy & breastfeeding safety ratings cover 3 of 3.
  • General safety write-ups exist for 3 of 3.
  • Remember: this measures how much safety information exists. Thin data is not the same as being safe.

Iron is generally well tolerated at recommended doses, though it can cause nausea, constipation, diarrhea, and abdominal pain — these are the most common side effects. Potassium is also generally well tolerated from food, but supplements can cause dangerously high blood levels in some people, especially those with kidney disease; a small number of people have experienced serious effects like irregular heartbeat or confusion from high potassium.

During pregnancy, potassium from a normal diet is fine, but use supplements only under medical guidance; iron is often recommended in pregnancy and is guided by your prenatal care provider. While breastfeeding, dietary potassium is appropriate, but check with your doctor before using supplements; iron is generally considered acceptable at appropriate doses, so talk it over with your provider.

Side effects, ingredient by ingredient Black Psyllium Potassium Iron

Meds to double-check

Moderate interaction found
Known Interaction Concern · database check
Moderate identified

The most serious documented interaction for these ingredients is Moderate. Check your medications for a personalized result.

Why this rating?
  • 3 of the 3 matched ingredients can interact with medications — Black Psyllium, Potassium, Iron.
  • The most serious interaction on file is rated Moderate.
  • Some involve high-stakes drug classes: heart-rhythm medications; lithium; Parkinson's medications.
  • For scale: 2,039 individual medications appear in the full list. A big number alone doesn't make a product dangerous — what matters is whether YOUR medication is on it, so run yours through the interaction checker on this page.

If you're on an ACE inhibitor, ARB, or potassium-sparing diuretic (for blood pressure or heart failure), check with your doctor first — these have a Moderate risk of high potassium levels when combined with this product. Also double-check before starting if you take any quinolone or tetracycline antibiotic, levothyroxine (thyroid medication), levodopa (Parkinson's treatment), a bisphosphonate (bone medication), methyldopa (blood pressure drug), mycophenolate mofetil (immunosuppressant), or penicillamine (Wilson's disease treatment) — iron in this capsule can reduce how much of these drugs your body absorbs, and you'll likely need to separate doses by a few hours.

Check your own medication Run your meds through the checker above

The bottom line

Scorecard at a glanceFully disclosed formula with clinical evidence supporting its stated purpose. Moderate medication interactions have been identified, and safety information is well characterized.

This product is designed to support iron stores and digestive health through soluble fiber. If you take blood pressure medications (especially ACE inhibitors, ARBs, or potassium-sparing diuretics), antibiotics, thyroid hormone, or any other medication, you'll need to check with your own doctor or pharmacist before starting — several of those drugs interact with potassium or iron and may need dose timing adjustments.

If you have kidney disease, talk to your provider first, since potassium supplements carry extra risk in that case.

Educational only — not medical advice; always confirm with your pharmacist. Our editorial policy · How we use AI

Assessment coverage: 3 of 3 active ingredients matched to our full ingredient reviews (monographs). Based on the product label dated Sep 24, 2019.

This Scorecard evaluates available label information, ingredient evidence, and known medication-safety considerations. It does not independently verify product identity, purity, potency, contamination, or manufacturing quality. How these ratings are computed

At a glance

General information

Key facts about Fiber Capsules, straight from the product label.

Brand Kirkland Signature
Net contents 360 Capsule(s)
Market status On market
Date entered into DSLD Sep 24, 2019
DSLD ID 207331
Product type Fiber And Other Nutrients
Supplement form Capsule
Dietary claims / uses Nutrient, All Other, Structure/Function
Intended target group(s) Children 4 or More Years of Age, Adult (18 - 50 Years), Gluten Free
From the label
Everything in this section is reproduced from the manufacturer’s own product label — it’s the label speaking, not HelloPharmacist. We show it so you can see exactly what the maker states; we don’t verify or endorse those statements.

Supplement Facts

The label details for Fiber Capsules by Kirkland Signature, sourced from the NIH Dietary Supplement Label Database.

Supplement Facts

Daily Value (DV) Target Group(s):
Adults and children 4 or more years of age
Minimum serving Sizes:
2 Capsule(s)
Maximum serving Sizes:
6 Capsule(s)
Servings per container
60
IngredientAmount% DV
Calories10 Calorie(s)--
Total Carbohydrates3 Gram(s)1%
Dietary Fiber3 Gram(s)11%
Potassium30 mg1%
Iron0.4 mg2%
Soluble Fiber2 Gram(s)--
Protein1 Gram(s)--

Other ingredients: Psyllium, Gelatin, Polysorbate 80, Caramel color, Acacia, Black Iron Oxide

Tap any ingredient to jump to its full detail below.

Label statements
These statements are the manufacturer’s wording, reproduced from the product label — the label is saying it, not HelloPharmacist. We don’t verify or endorse them.
General Statements

Compare to Metamucil Capsules

Helps lower cholesterol to promote heart health Helps maintain healthy blood sugar levels as part of your diet Promotes digestive health

Formula

100% natural Psyllium Husk Fiber

Diets low in saturated fat and cholesterol that include 7 grams of soluble fiber per day from psyllium seed husk, may reduce the risk of heart disease by lowering cholesterol levels. One serving provides 24 grams of soluble fiber.

FDA Statement of Identity

Fiber Supplement

FDA Disclaimer Statement

These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure or prevent any disease.

Precautions

Do not use if printed seal under cap is torn or missing.

Consult a doctor if you are considering use of the product as part of a cholesterol-lowering program.

Notice: Take this product with at least 8 oz (a full glass) of liquid. Taking this product without enough liquid may cause choking. Do not take if you have difficulty swallowing.

Allergy alert: May cause allergic reaction in people sensitive to ingested or inhaled psyllium.

Ask a doctor before use if you have abdominal pain, nausea or vomiting or experience a sudden change in bowel habits persisting for 2 weeks. Stop use and ask a doctor if constipation lasts more than 7 days or rectal bleeding occurs. These may be signs of a serious condition.

Bulk-forming fibers like psyllium husk may affect how well other medicines work. If you are taking a prescription medicine by mouth, take this product at least 2 hours before or 2 hours after the prescribed medicine.

Keep out of reach of children.

Brand IP Statement(s)

This product is not manufactured of distributed by Proctor & Gamble, the distributor of METAMUCIL Capsules.

Formulation

Gluten free (<20 ppm)

Suggested/Recommended/Usage/Directions

Directions: Adults 12 years & over: 2-6 capsules for increasing daily fiber intake and 6 capsules for cholesterol-lowering program. Take this product with 8 oz of water (swallow 1 capsule at a time) up to 3 times daily. Children under 12 years: Consult a doctor New users: Start with 1 serving per day; gradually increase up to 3 servings per day as necessary. You may initially experience changes in bowel habits or minor bloating as your body adjusts to increased fiber intake. Notice: Take this product with at least 8 oz (a full glass) of liquid. Taking this product without enough liquid may cause choking. Do not take if you have difficulty swallowing.

Storage

Store at room temperature. Protect contents from moisture.

Is this label outdated? Report a formula or label change and our pharmacy team will review it.

See for yourself

Fiber Capsules by Kirkland Signature label

The label scan from the NIH Dietary Supplement Label Database. Tap to enlarge.

Fiber Capsules label

Label images are published by the NIH Dietary Supplement Label Database for the version of this product on file. Always read your actual product label.

View the full label (PDF)
What’s inside

The Ingredients in Fiber Capsules by Kirkland Signature

These are the 4 active ingredients this product is made of. Select any to open its full monograph.

Serving size2 Capsule(s) Dosage formCapsule Servings per container60 Amounts shown are per serving.

Most supplement products combine several ingredients, and a medication can interact with the product through any one of them. Each ingredient below shows whether it has known drug interactions.

Dietary Fiber

Interacts with
2,025 drugs
3 Gram(s) per serving

Black psyllium is a soluble-fiber supplement made from the seeds of a Plantago plant, used mostly to ease constipation and support digestive health. I...

Dietary Fiber monograph & interactions
  • › Soluble Fiber

Potassium

Interacts with
62 drugs
30 mg per serving

Potassium is an essential mineral your body needs for nerve signals, muscle function, and a steady heartbeat, and most people get enough from a balanc...

Potassium monograph & interactions

Iron

Interacts with
80 drugs
0.4 mg per serving

Iron is an essential mineral your body needs to make hemoglobin and carry oxygen in the blood. Supplements are mainly useful for treating or preventin...

Iron monograph & interactions

Protein

1 Gram(s) per serving

Other (inactive) ingredients: Psyllium, Gelatin, Polysorbate 80, Caramel color, Acacia, Black Iron Oxide. These complete the product’s ingredient list but are not active constituents.

Interaction report

Fiber Capsules by Kirkland Signature Drug Interactions

Want to check YOUR meds against Fiber Capsules?

Ask about interactions with your drugs in plain English — “Can I take it with lisinopril?” — and we find you the answer in seconds, ingredient by ingredient.

 Go to the checker
2,038Drugs
164 Moderate 1,874 Minor

Ingredients driving the most interactions

Dietary Fiber 2,025
Iron 80
Potassium 62

Individual Drug Interactions

The ingredients in Fiber Capsules interact with 2,038 drugs. Click any drug to see the details.

3 of the 4 ingredients in Fiber Capsules interact with drugs. Each result below shows which ingredient is responsible. Dietary Fiber Iron Potassium

Each ingredient & the kinds of drugs it affects

For each ingredient in Fiber Capsules with known interactions, here are the types of medications they can affect. Open any type for the detail — or search your exact drug in the checker above.

Dietary Fiber7 drug types · 2,025 drugs

Carbamazepine (Tegretol)

Theoretically, black psyllium might reduce the effects of carbamazepine and increase the risk for convulsions.
Theoretically, black psyllium might reduce carbamazepine absorption. A preliminary study using blond psyllium reported decreased carbamazepine bioavailability due to binding of the drug to psyllium, as well as reduction of available fluid in the gut for dissolution of the drug. This interaction may also occur with black psyllium.

Likelihood Probable Evidence D
Lithium

Theoretically, taking black psyllium at the same time as lithium might reduce lithium absorption.
The fiber in black psyllium might reduce lithium absorption and plasma levels. Some case reports describe a reduction in plasma lithium levels with concomitant administration of blond psyllium. This was reversed when psyllium was stopped. This interaction may also occur with black psyllium.

Likelihood Probable Evidence D
Metformin (Glucophage)

Theoretically, black psyllium might increase the therapeutic and adverse effects of metformin.
Animal research shows that concurrent consumption of blond psyllium with metformin slows and increases the absorption of metformin. This interaction may also occur with black psyllium. To avoid changes in absorption, take psyllium 30-60 minutes after metformin.

Likelihood Possible Evidence D
Olanzapine (Zyprexa)

Theoretically, taking black psyllium at the same time as olanzapine might reduce olanzapine absorption.
The fiber in black psyllium might decrease the absorption of olanzapine. A single case report describes a reduction in the effectiveness of olanzapine when it was concomitantly administered with an unspecified type of psyllium 3 grams orally twice daily. This effect was reversed when psyllium was stopped.

Likelihood Possible Evidence D
Digoxin (Lanoxin)

Theoretically, taking black psyllium at the same time as digoxin might reduce digoxin absorption and decrease digoxin levels.
Psyllium might bind digoxin in the gut. However, some clinical evidence suggests that psyllium does not impact digoxin absorption.

Likelihood Unlikely Evidence B
Ethinyl Estradiol

Theoretically, taking black psyllium at the same time as ethinyl estradiol might alter levels of estradiol.
Concurrent use of blond psyllium with ethinyl estradiol results in a slight increase in the extent of ethinyl estradiol absorption and a slower rate of absorption. This is unlikely to be clinically significant.

Likelihood Unlikely Evidence D
Oral Drugs

Theoretically, psyllium might increase, decrease, or have no effect on the absorption of oral drugs.
Psyllium seems to have variable effects on drug absorption. To avoid changes in absorption, take psyllium 30-60 minutes after oral medications. Animal research shows that blond psyllium delays and increases the absorption of metformin and ethinyl estradiol. Case reports and animal research suggest that blond psyllium might reduce absorption of lithium, digoxin, olanzapine, and carbamazepine. Finally, some pharmacokinetic studies show that psyllium does not affect the absorption of levothyroxine or warfarin. Although many of these studies evaluated blond psyllium, the fiber content in black psyllium may have similar effects.

Likelihood Possible Evidence B
Dietary Fiber full monograph & interactions

Iron13 drug types · 80 drugs

Bictegravir/Emtricitabine/Tenofovir Alafenamide (Biktarvy)

Iron might decrease levels of bictegravir/emtricitabine/tenofovir alafenamide by reducing its absorption when taken in a fasting state.
Advise patients that bictegravir/emtricitabine/tenofovir alafenamide and iron can be taken together if taken with food. However, if taken on an empty stomach, bictegravir/emtricitabine/tenofovir alafenamide should not be taken with, or 2 hours after, iron containing products.

Likelihood Probable Evidence D
Bisphosphonates

Iron reduces the absorption of bisphosphonates.
Advise patients that doses of bisphosphonates should be separated by at least two hours from doses of all other medications, including supplements such as iron. Divalent cations, including iron, can decrease absorption of bisphosphonates by forming insoluble complexes in the gastrointestinal tract.

Likelihood Probable Evidence D
Denosumab (Prolia, Others)

Administration of intravenous iron within one month of denosumab administration might increase the risk of severe hypophosphatemia and hypocalcemia.
A case of severe hypocalcemia (albumin corrected calcium 6.88 mg/dL, ionized calcium 3.68 mg/dL) and hypophosphatemia (<0.5 mg/dL) with respiratory acidosis, QT interval prolongation, and nonsustained ventricular tachycardia was reported in a 76-year-old male who had received an iron polymaltose infusion within 2 weeks of a subcutaneous injection of denosumab. Serum parathyroid hormone was also elevated (348 pg/mL). Subsequent iron infusions with iron polymaltose and ferric carboxymaltose were followed by transient hypophosphatemia, but without hypocalcemia. Additionally, a literature review describes 6 additional cases of hypophosphatemia and hypocalcemia in patients 52-92 years of age who had been administered intravenous iron as either ferric carboxymaltose or iron polymaltose and subcutaneous denosumab within 1-4 weeks of each other.

Likelihood Possible Evidence D
Dolutegravir (Tivicay)

Iron might decrease dolutegravir levels by reducing its absorption.
Advise patients to take dolutegravir at least 2 hours before or 6 hours after taking iron. Pharmacokinetic research shows that iron can decrease the absorption of dolutegravir from the gastrointestinal tract through chelation. When taken under fasting conditions, a single dose of ferrous fumarate 324 mg orally along with dolutegravir 50 mg reduces overall exposure to dolutegravir by 54%.

Likelihood Probable Evidence B
Integrase Inhibitors

Theoretically, taking iron along with integrase inhibitors might decrease the levels and clinical effects of these drugs.
Iron is a divalent cation. There is concern that iron may decrease the absorption of integrase inhibitors from the gastrointestinal tract through chelation. One pharmacokinetic study shows that iron can decrease blood levels of the specific integrase inhibitor dolutegravir through chelation. Also, other pharmacokinetic research shows that other divalent cations such as calcium can decrease the absorption and levels of some integrase inhibitors through chelation.

Likelihood Possible Evidence D
Levodopa

Iron might decrease levodopa levels by reducing its absorption.
Advise patients to separate doses of levodopa and iron as much as possible. There is some evidence in healthy people that iron forms chelates with levodopa, reducing the amount of levodopa absorbed by around 50%. The clinical significance of this hasn't been determined.

Likelihood Probable Evidence B
Levothyroxine (Synthroid, Others)

Iron might decrease levothyroxine levels by reducing its absorption.
Advise patients to separate levothyroxine and iron doses by at least 2 hours. Iron can decrease the absorption and efficacy of levothyroxine by forming insoluble complexes in the gastrointestinal tract.

Likelihood Probable Evidence B
Methyldopa (Aldomet)

Iron might decrease methyldopa levels by reducing its absorption.
Advise patients to separate methyldopa and iron doses by at least 2 hours. Iron can decrease the absorption of methyldopa from the gastrointestinal tract through chelation, resulting in increases in blood pressure.

Likelihood Probable Evidence B
Mycophenolate Mofetil (Cellcept)

Theoretically, iron might decrease mycophenolate mofetil levels by reducing its absorption.
Advise patients to take iron 4-6 hours before, or 2 hours after, mycophenolate mofetil. It has been suggested that a decrease of absorption is possible, probably by forming nonabsorbable chelates. However, mycophenolate pharmacokinetics are not affected by iron supplementation in available clinical research.

Likelihood Unlikely Evidence D
Penicillamine (Cuprimine, Depen)

Iron might decrease penicillamine levels by reducing its absorption.
Advise patients to separate penicillamine and iron doses by at least 2 hours. Oral iron supplements can reduce absorption of penicillamine by 30% to 70%, probably due to chelate formation. In people with Wilson's disease, this interaction has led to reduced efficacy of penicillamine.

Likelihood Probable Evidence D
Quinolone Antibiotics

Iron might decrease levels of quinolone antibiotics by reducing their absorption.
Advise patients to separate quinolone antibiotics and iron doses by at least 2 hours. Iron decreases the absorption of quinolones due to formation of insoluble complexes in the gastrointestinal tract.

Likelihood Probable Evidence D
Tetracycline Antibiotics

Iron might decrease levels of tetracycline antibiotics by reducing their absorption.
Advise patients to take iron at least 2 hours before or 4 hours after tetracycline antibiotics. Concomitant use can decrease absorption of tetracycline antibiotics from the gastrointestinal tract by 50% to 90%.

Likelihood Probable Evidence D
Chloramphenicol

Theoretically, taking chloramphenicol with iron might reduce the response to iron therapy in iron deficiency anemia.
Chloramphenicol interferes with erythrocyte maturation. However, since chloramphenicol isn't usually taken for prolonged periods, this isn't likely to be clinically significant.

Likelihood Unlikely Evidence D
Iron full monograph & interactions

Potassium3 drug types · 62 drugs

Ace Inhibitors (Aceis)

Using ACEIs with high doses of potassium increases the risk of hyperkalemia.
ACEIs block the actions of the renin-angiotensin-aldosterone system and reduce potassium excretion. Concomitant use of these drugs with potassium supplements increases the risk of hyperkalemia. However, concomitant use of these drugs with moderate dietary potassium intake (about 3775-5200 mg daily) does not increase serum potassium levels.

Likelihood Likely Evidence C
Angiotensin Receptor Blockers (Arbs)

Using ARBs with high doses of potassium increases the risk of hyperkalemia.
ARBs block the actions of the renin-angiotensin-aldosterone system and reduce potassium excretion. Concomitant use of these drugs with potassium supplements increases the risk of hyperkalemia. However, concomitant use of these drugs with moderate dietary potassium intake (about 3775-5200 mg daily) does not increase serum potassium levels.

Likelihood Likely Evidence C
Potassium-Sparing Diuretics

Concomitant use increases the risk of hyperkalemia.
Using potassium-sparing diuretics with potassium supplements increases the risk of hyperkalemia.

Likelihood Likely Evidence C
Potassium full monograph & interactions
The maker

Brand information

Manufacturer and brand details for Fiber Capsules, from the product label.

Kirkland Signature

See all Kirkland Signature products
Name
Costco Wholesale Corporation
Street Address
P.O. Box 34535
City
Seattle
State
WA
ZipCode
98124-1535
Web Address
www.costco.com
Pharmacist Counseling Corner

Fiber Capsules by Kirkland Signature: Common Questions

Does Fiber Capsules by Kirkland Signature interact with any medications?
Yes. Based on its ingredients, Fiber Capsules has a known interaction with 2,038 medications. Use the checker to see how it interacts with a specific drug.
How can one product interact with so many drugs?
Fiber Capsules contains 4 active ingredients, and an interaction can come from any of them. We check every ingredient, combine the results into one list per medication, and show which ingredient and mechanism is responsible.
Where does this information come from?
The product label data comes from the NIH Dietary Supplement Label Database (DSLD); the interaction data is built on the Natural Medicines database and reviewed by HelloPharmacist pharmacists.
Does this product have iron in it?
Yes, it does. Iron helps treat iron deficiency anemia and is effective for anemia of chronic disease and pregnancy-related iron deficiency. It's generally well tolerated, though it can cause constipation, diarrhea, nausea, or belly pain in some people.
Can I take this while I'm pregnant?
The potassium and iron in this product are rated likely safe in pregnancy, but dosing should be guided by your prenatal care provider — don't start a supplement on your own. Talk with your doctor first.
Can I take this while breastfeeding?
Potassium from diet is fine, but check with your doctor before taking a potassium supplement while nursing. Iron is generally acceptable at appropriate doses — confirm the dose is right for you with your healthcare provider.
What's the soluble fiber in here for?
Soluble fiber absorbs water in your digestive tract and helps soften stool. We don't hold effectiveness data on this ingredient, so I can't tell you what the evidence shows for how well it works.
What are the most common side effects?
Iron can cause nausea, constipation, diarrhea, and belly pain. Potassium can cause similar stomach issues — nausea, vomiting, belly pain, and diarrhea. Most people tolerate it fine at recommended doses.
Is this safe if I have kidney disease?
If you have kidney disease, talk with your doctor before taking this product. Potassium supplements can build up to dangerously high levels in people with kidney problems, so your provider needs to weigh whether it's right for you.

Written and reviewed by the HelloPharmacist editorial staff. Our editorial policy

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Label information is sourced from the NIH Dietary Supplement Label Database and reflects the product version on file; always read your actual product label. This page is for education only and is not a substitute for professional medical advice. Confirm with your pharmacist or doctor before combining supplements and medications.

Fiber Capsules label
Go deeper

The Full Monographs Behind Fiber Capsules’s Ingredients

Every ingredient we hold a full HelloPharmacist monograph for — uses, evidence, safety, and the complete interaction list.

Herb & supplement monograph

Black Psyllium

Interacts with 2,025 drugs

Black psyllium is a soluble-fiber supplement made from the seeds of a Plantago plant, used mostly to ease constipation and support digestive health. It is best known and most studied for rel...

Read the full Black Psyllium monograph → Herb & supplement monograph

Potassium

Interacts with 62 drugs

Potassium is an essential mineral your body needs for nerve signals, muscle function, and a steady heartbeat, and most people get enough from a balanced diet rich in fruits and vegetables. P...

Read the full Potassium monograph → Herb & supplement monograph

Iron

Interacts with 80 drugs

Iron is an essential mineral your body needs to make hemoglobin and carry oxygen in the blood. Supplements are mainly useful for treating or preventing iron deficiency and iron-deficiency an...

Read the full Iron monograph →
Sources

Sources & How We Checked

Fiber Capsules's label data comes from the NIH Dietary Supplement Label Database; the ingredient interaction data is from the Natural Medicines database, reviewed by our pharmacists.

Content is written and reviewed by licensed HelloPharmacist pharmacists. See our data sources and editorial standards for how this information is built and checked.

The 102 references behind this product’s interaction data

Every citation that drives the interaction findings for this product’s ingredients, from the evidence-graded Natural Medicines (TRC Healthcare) database. Open an ingredient to browse its citations — links open the study on PubMed or the publisher’s site.

Black Psyllium 18 references
  1. McGuffin M, Hobbs C, Upton R, Goldberg A, eds. American Herbal Products Association's Botanical Safety Handbook. Boca Raton, FL: CRC Press, LLC 1997.
  2. Gruenwald J, Brendler T, Jaenicke C. PDR for Herbal Medicines. 1st ed. Montvale, NJ: Medical Economics Company, Inc., 1998.
  3. Covington TR, et al. Handbook of Nonprescription Drugs. 11th ed. Washington, DC: American Pharmaceutical Association, 1996.
  4. Etman M. Effect of a bulk forming laxative on the bioavailablility of carbamazepine in man. Drug Dev Ind Pharm 1995;21:1901-6.
  5. Perlman BB. Interaction between lithium salts and ispaghula husk. Lancet 1990;335:416.
  6. Vaswani SK, Hamilton RG, Valentine MD, Adkinson NF. Psyllium laxative-induced anaphylaxis, asthma, and rhinitis. Allergy 1996;51:266-8. PubMed
  7. Lantner RR, Espiritu BR, Zumerchik P, Tobin MC. Anaphylaxis following ingestion of a psyllium-containing cereal. JAMA 1990;264:2534-6. DOI
  8. Kaplan MJ. Anaphylactic reaction to "Heartwise." N Engl J Med 1990;323:1072-3. DOI
  9. Nordstrom M, Melander A, Robertsson E, Steen B. Influence of wheat bran and of a bulk-forming ispaghula cathartic on the bioavailability of digoxin in geriatric in-patients. Drug Nutr Interact 1987;5:67-9..
  10. Robinson DS, Benjamin DM, McCormack JJ. Interaction of warfarin and nonsystemic gastrointestinal drugs. Clin Pharmacol Ther 1971;12:491-5. PubMed
  11. Garcia JJ, Fernandez N, Diez MJ, et al. Influence of two dietary fibers in the oral bioavailability and other pharmacokinetic parameters of ethinyloestradiol. Contraception 2000;62:253-7. PubMed
  12. Fernandez N, Lopez C, Díez R, et al. Drug interactions with the dietary fiber Plantago ovata husk. Expert Opin Drug Metab Toxicol 2012;8(11):1377-86.
  13. Semen plantaginis in: WHO Monographs on Selected Medicinal Plants, volume 1. World Health Organization, Geneva, 1999. Available at http://apps.who.int/medicinedocs/en/d/Js2200e/. Accessed November 26, 1026.
  14. Code of Federal Regulations, Title 21 (21CFR 101.17). Food labeling warning, notice, and safe handling statements. Available at www.ecfr.gov/cgi-bin/text-idx?SID=20f647d3b74161501f46564b915b4048&mc=true&node=se21.2.101_117&rgn=div8. Accessed December 3, 2
  15. Code of Federal Regulations, Title 21 (21CFR 201.319). Specific labeling requirements - water-soluble gums, hydrophilic gums, and hydrophilic mucilloids. Available at www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfcfr/CFRSearch.cfm?fr=201.319. Accessed Dece
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See these in context on the Iron monograph →

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