Drug & supplement interaction

Phenylalanine + Diltiazem

Diltiazem brand names: Cardizem, Cardizem SR, Diltiaz SR, Cardizem CD, Tiazac, and others.

No Known Interaction
Medically reviewed by · Updated July 2026 · Pair last verified Sep 20, 2026
The bottom line No known interaction

We found no known interaction between Phenylalanine and Diltiazem in our sources. Absence of a documented interaction is not the same as proven safety, and does not guarantee one does not exist. Do not start, stop, or change a medication or supplement based on this page; confirm with your pharmacist or doctor first. Additionally, although there is no interaction, the two are separately associated with some of the same side effects, which would make a new symptom harder to attribute to one or the other.

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Interaction report

Phenylalanine and Diltiazem

We checked these two against our interaction database. Here is what we found.

No Known Interaction Found

We did not find a documented interaction between Diltiazem and Phenylalanine in our sources. This does not guarantee that the combination is completely free of risk — there is often limited research on supplements and herbal products, especially around drug interactions.

If you decide to take both, the practical routine is simple: add one new thing at a time and note the date you started it. If the combination is going to disagree with you, it usually shows in the first days to a couple of weeks — and a start date turns “I’ve been feeling off” into something a pharmacist can actually work with. Do not stop a prescribed medicine on your own.

Always talk with your pharmacist or doctor before making changes to your medications or supplements, particularly if you take several medications or have an ongoing health condition.

Phenylalanine safety at a glance

Phenylalanine overall Use caution Generally tolerated in food amounts, but supplements should be used carefully and not by people with PKU.
Pregnancy Likely Safe When L-phenylalanine is consumed in amounts typically found in foods by pregnant patients with normal phenylalanine metabolism. Read the full pregnancy detail
Breastfeeding Likely Safe When L-phenylalanine is consumed in amounts typically found in foods by breast-feeding patients with normal phenylalanine metabolism. There is insufficient reli... Read the full breastfeeding detail

Pregnancy & breastfeeding ratings are from Natural Medicines (Therapeutic Research Center) — the full detail is in the Special populations card below. Safety guidance is general; always confirm with your pharmacist or doctor for your situation.

Still wondering about combining Diltiazem and Phenylalanine? Ask Dr. Brian Staiger, PharmD, BCPS directly ›
How we checked

Why No Interaction Is Listed for Diltiazem

Interactions in our licensed database are recorded against drug classes, not just single drug names. Diltiazem is filed under 9 classifications — including calcium channel blockers, cytochrome p450 1a2 (cyp1a2) inhibitors, cytochrome p450 3a4 (cyp3a4) inhibitors. Phenylalanine has documented interactions with 3 drug classes — the most serious being levodopa (major), baclofen (moderate) and monoamine oxidase inhibitors (maois) (moderate) — and none of Diltiazem’s classifications are among them.

Classification data comes from our licensed Natural Medicines dataset. A medication can belong to classes beyond those recorded there, so this describes how the check was run — it is not a safety guarantee.

Keep it in perspective

Phenylalanine Does Interact With Other Medications

Phenylalanine has 5 major, 11 moderate known drug interactions in our database — Diltiazem isn’t one of them. If you take anything else, check it against the full list below.

Major 5 Moderate 11 No known interaction 2,814 (incl. Diltiazem)
Taking anything else?

Check Another of Your Medications Against Phenylalanine

Covers the 2,830 medications we’ve checked against Phenylalanine. Pick one to open its full report.

What to actually watch for

No Interaction Found — But Data Suggests Phenylalanine and Diltiazem May Share Several of the Same Side Effects

This is not an interaction. Neither one changes how your body handles the other, and nothing here says they react together. What our sources do show is that each has separately been linked to the effects below — so taking both can make those effects more likely to turn up, and harder to pin on one or the other if they do.

  • Drowsiness / Trouble sleeping Additional detail

    Phenylalanine

    “Orally, L-phenylalanine has been associated with anxiety, insomnia, and, more rarely, hypomania. DL-phenylalanine has been associated with fatigue and sedation.”

    Phenylalanine · Natural Medicines adverse-effect record, reported in anxiety.

    3 sources cited
    1. 68827. Sabelli, H. C., Fawcett, J., Gusovsky, F., Javaid, J. I., Wynn, P., Edwards, J., Jeffriess, H., and Kravitz, H. Clinical studies on the phenylethylamine hypothesis of affective disorder: urine and blood phenylacetic acid and phenylalanine dietary supplem
    2. 68829. Kravitz, H. M., Sabelli, H. C., and Fawcett, J. Dietary supplements of phenylalanine and other amino acid precursors of brain neuroamines in the treatment of depressive disorders. J Am Osteopath.Assoc 1984;84(1 Suppl):119-123. DOI
    3. 9951. Wood DR, Reimherr FW, Wender PH. Treatment of attention deficit disorder with DL-phenylalanine. Psychiatry Res 1985;16:21-6.. PubMed

    Diltiazem

    Listed in the ADVERSE REACTIONS section of the FDA label for Diltiazem. Read the label on DailyMed ›

  • Tiredness reported in 7.9% of FDA reports for Diltiazem Additional detail

    Phenylalanine

    Recorded for Phenylalanine in the same Natural Medicines adverse-effect record shown under Drowsiness / Trouble sleeping above.

    Diltiazem

    Listed in the ADVERSE REACTIONS section of the FDA label for Diltiazem, and in voluntary FDA adverse-event reports. Read the label on DailyMed ›

  • Constipation / Nausea reported in 7.1% of FDA reports for Diltiazem Additional detail

    Phenylalanine

    “Orally, constipation, heartburn, and nausea has been reported in some patients taking phenylalanine.”

    Phenylalanine · Natural Medicines adverse-effect record, reported in constipation.

    4 sources cited
    1. 2463. Siddiqui AH, Stolk LM, Bhaggoe R, et al. L-phenylalanine and UVA irradiation in the treatment of vitiligo. Dermatology 1994;88:215-8. PubMed
    2. 68827. Sabelli, H. C., Fawcett, J., Gusovsky, F., Javaid, J. I., Wynn, P., Edwards, J., Jeffriess, H., and Kravitz, H. Clinical studies on the phenylethylamine hypothesis of affective disorder: urine and blood phenylacetic acid and phenylalanine dietary supplem
    3. 68829. Kravitz, H. M., Sabelli, H. C., and Fawcett, J. Dietary supplements of phenylalanine and other amino acid precursors of brain neuroamines in the treatment of depressive disorders. J Am Osteopath.Assoc 1984;84(1 Suppl):119-123. DOI
    4. 68830. Mann, J., Peselow, E. D., Snyderman, S., and Gershon, S. D-phenylalanine in endogenous depression. Am.J.Psychiatry 1980;137(12):1611-1612. PubMed

    Diltiazem

    Listed in the ADVERSE REACTIONS section of the FDA label for Diltiazem, and in voluntary FDA adverse-event reports. Read the label on DailyMed ›

  • Dizziness reported in 6.3% of FDA reports for Diltiazem Additional detail

    Phenylalanine

    “Orally, headaches, which are typically transient and do not require treatment or dosage reduction, have been reported during the first 10 days of treatment with L-, D-, and DL-phenylalanine. Transient vertigo has also been reported with D- and DL-phenylalanine.

    In patients with Parkinson disease, taking DL-phenylalanine, especially in high doses, interferes with levodopa transport into the brain, causing increased ri…”

    Phenylalanine · Natural Medicines adverse-effect record, reported in myasthenia gravis.

    6 sources cited
    1. 68795. Fischer, E., Heller, B., Nachon, M., and Spatz, H. Therapy of depression by phenylalanine. Preliminary note. Arzneimittelforschung. 1975;25(1):132.
    2. 68825. Beckmann, H., Strauss, M. A., and Ludolph, E. Dl-phenylalanine in depressed patients: an open study. J.Neural Transm. 1977;41(2-3):123-134. PubMed
    3. 68827. Sabelli, H. C., Fawcett, J., Gusovsky, F., Javaid, J. I., Wynn, P., Edwards, J., Jeffriess, H., and Kravitz, H. Clinical studies on the phenylethylamine hypothesis of affective disorder: urine and blood phenylacetic acid and phenylalanine dietary supplem
    4. 68829. Kravitz, H. M., Sabelli, H. C., and Fawcett, J. Dietary supplements of phenylalanine and other amino acid precursors of brain neuroamines in the treatment of depressive disorders. J Am Osteopath.Assoc 1984;84(1 Suppl):119-123. DOI
    5. 3291. Nutt JG, Woodward WR, Hammerstad JP, et al. The "on-off" phenomenon in Parkinson's disease. Relation to levodopa absorption and transport. N Engl J Med 1984;310:483-8. PubMed
    6. 3292. Baruzzi A, Contin M, Riva R, et al. Influence of meal ingestion time on pharmacokinetics of orally administered levodopa in parkinsonian patients. Clin Neuropharmacol 1987;10:527-37. PubMed

    Diltiazem

    Listed in the ADVERSE REACTIONS section of the FDA label for Diltiazem, and in voluntary FDA adverse-event reports. Read the label on DailyMed ›

  • Itching Additional detail

    Phenylalanine

    “Topically, erythema, itching, and burning have been reported in some patients using an undecylenoyl phenylalanine 2% cream for treatment of age spots.”

    Phenylalanine · Natural Medicines adverse-effect record, reported in erythema.

    1 source cited
    1. 92704. Katoulis AC, Alevizou A, Bozi E, et al. A randomized, double-blind, vehicle-controlled study of a preparation containing undecylenoyl phenylalanine 2% in the treatment of solar lentigines. Clin Exp Dermatol 2010;35(5):473-6. PubMed

    Diltiazem

    Listed in the ADVERSE REACTIONS section of the FDA label for Diltiazem. Read the label on DailyMed ›

  • Headache reported in 5.9% of FDA reports for Diltiazem Additional detail

    Phenylalanine

    Recorded for Phenylalanine in the same Natural Medicines adverse-effect record shown under Drowsiness / Trouble sleeping above.

    Diltiazem

    Listed in the ADVERSE REACTIONS section of the FDA label for Diltiazem, and in voluntary FDA adverse-event reports. Read the label on DailyMed ›

Reported for Phenylalanine but not commonly for Diltiazem: heartburn, mania or an unusually elevated mood, anxiety or restlessness and tremor or shakiness.

Effects for Diltiazem come from the ADVERSE REACTIONS section of its FDA label; effects for Phenylalanine come from the Natural Medicines adverse-effect record, quoted above. Where a percentage is shown, it is the share of 34,400 voluntary FDA FAERS reports for Diltiazem that mention that effect — a reporting proportion, not an incidence rate and not evidence the drug caused it. Wording is matched across sources by our own clinical vocabulary, so treat this as a prompt to pay attention, not a diagnosis.

Special populations

Phenylalanine in Pregnancy & Breastfeeding

Pregnancy Likely Safe

When L-phenylalanine is consumed in amounts typically found in foods by pregnant patients with normal phenylalanine metabolism.

Pregnancy Unsafe

When L-phenylalanine is consumed in amounts typically found in foods by pregnant patients with high serum phenylalanine concentrations, such as those with phenylketonuria (PKU). Serum levels of phenylalanine greater than 360 micromol/L increase the risk of birth defects. Experts recommend that patients with high phenylalanine serum concentrations follow a low phenylalanine diet for at least 20 weeks prior to conception to decrease the risk for birth defects. There is insufficient reliable information available about the safety of L-phenylalanine when taken by mouth in large doses during pregnancy; avoid using. There is insufficient reliable information available about the safety of oral D-phenylalanine during pregnancy; avoid using.

Lactation Likely Safe

When L-phenylalanine is consumed in amounts typically found in foods by breast-feeding patients with normal phenylalanine metabolism. There is insufficient reliable information available about the safety of L-phenylalanine when taken by mouth in medicinal amounts during lactation; avoid using. There is insufficient reliable information available about the safety of oral D-phenylalanine during lactation; avoid using.

DISCLAIMER: This tool is intended for informational purposes only, and should not be interpreted as specific medical advice. Patients should consult with a qualified healthcare provider before making decisions about therapies and/or health conditions.© 2026 Therapeutic Research Center

From our Q&A

Questions About Phenylalanine or Diltiazem

These are real questions from readers, answered by the pharmacists at HelloPharmacist — written by a person, not generated from a database. Each one covers Phenylalanine or Diltiazem, not necessarily both, but if one is close to what you were about to ask it will usually go further than a database entry can:

Not quite your question? Ask our pharmacists directly — it is free, there is no account required, and answers usually come back within 24 hours.

Everything else you take

See Every Supplement That Interacts With Diltiazem

This page covers Phenylalanine only. If you take other supplements — a multivitamin, a sleep aid, something for joints — the Diltiazem page lists 260 supplements it has a documented interaction with, worst first, each one explained.

The most serious there right now: Activated Charcoal (major), American Hellebore (major), American Mistletoe (major), Coleus (major) and Grapefruit (major).

By severity: 7 major · 214 moderate · 39 minor.

From our pharmacist’s overview on the Diltiazem page

“Someone on diltiazem (Cardizem, Tiazac) is usually managing blood pressure, angina, or a fast heart rhythm, so this list deserves genuine care. Seven of the 260 ingredients on file carry a major rating, and two of those are things people actually buy: grapefruit, which blocks the enzyme that clears diltiazem and can push its blood leve…”

Reviewed by 2 · Jul 18, 2026

Open the Diltiazem interaction page

One more smart check

Taking Phenylalanine as Part of a Dietary Supplement Product? Check It for Interactions

We hold 1,990 real supplement products containing Phenylalanine, each one broken down to its full ingredient list — and combination products often carry several ingredients besides Phenylalanine. Even though Phenylalanine itself has no known interaction with Diltiazem, another ingredient on your label might. Find your product and we’ll check every ingredient on its label:

How much people look this up

Where Phenylalanine and Diltiazem Sit Against Everything Else

Out of every supplement and medication people look up on HelloPharmacist, here is where these two land this week — and which way each has been moving.

Supplement Phenylalanine
Top 10%

#134 of 1,452 supplements looked up here in the last 7 days

29 days ago today
Medication Diltiazem
Top 5%

#134 of 2,830 medications looked up here in the last 7 days

29 days ago today

Position among everything looked up on this site, not a measure of how commonly either one is used or how important it is. A supplement can climb this list because of a news story. It counts every request to the page, automated ones included, and nothing here identifies anyone.

Where does our data come from?
  • Source: the Natural Medicines (TRC Healthcare) clinical database — the evidence-graded professional reference for herb & supplement interactions.
  • Citation library: 103,510 references behind our interaction content, 65,311 of them resolved to verified PubMed/DOI sources. See every Phenylalanine reference on this page ↓
  • Freshness: we automatically re-check this combination every week — this pair was last verified September 20, 2026. The Phenylalanine and Diltiazem records this page draws on were last updated July 11, 2026.
  • What “no known interaction” means: no documented interaction in the sources above — not a guarantee of safety. Supplements are studied far less than prescription drugs, so we say exactly what the evidence supports, and nothing more.
Data structure & sourcing designed and verified by .
Supplement

About Phenylalanine

What is Phenylalanine?

  • Depression and mood
  • Skin condition vitiligo
  • Pain conditions
  • Attention and focus
  • Appetite control

Phenylalanine is an essential amino acid the body uses to make brain chemicals like dopamine and norepinephrine. Some people take it for mood, vitiligo, or pain, but the evidence is mostly limited or mixed. People with the genetic condition PKU must strictly avoid it.

Amino acid (not a plant part) · Capsules, tablets, powder

An interaction check asks whether these two react with each other — it does not ask whether Phenylalanine is a good idea for someone in your situation. Here is what our Phenylalanine record says about it on its own.

Phenylalanine: dosing

There is no well-established standard supplement dose for phenylalanine, and the right amount may differ depending on the form (L-, D-, or DL-) and the reason for use.If you choose to use a product, follow the directions on the label and do not exceed the recommended amount.

From our Phenylalanine monograph — the full record covers uses, side effects and every documented interaction.

Read the full Phenylalanine monograph & all interactions
Medication

About Diltiazem

What Is Diltiazem Used For?

Brand names: Cardizem, Cardizem SR, Diltiaz SR, Cardizem CD, Tiazac, Diltiaz XT, Diltiaz CD, Dilacor, Diltiaz, Cardizem LA, Dilacor XR, Cartia XT, Tiamate, Adizem-SR, Adizem-XL, Angitil SR, Angitil XL, Calcicard CR, Dilcardia SR, Dilzem SR, Dilzem XL, Slozem, Tildiem LA, Tildiem Retard, Viazem XL, Zemtard 120XL, Zemtard 180XL, Zemtard 240XL, Zemtard 300XL

Oral extended-release forms of diltiazem — including Matzim LA, Cardizem LA, Cartia, Cartia XT, Tiadylt ER, and various generic capsules and tablets — are used to treat high blood pressure (hypertension) and chronic stable angina (chest pain brought on by activity). Some capsule formulations, including Cartia, Cartia XT, and certain generics, are also approved for angina due to coronary artery spasm (sudden chest pain caused by a sudden tightening of a coronary artery).

Injectable forms of diltiazem are used in hospital settings to temporarily slow a dangerously fast heart rate in atrial fibrillation or atrial flutter (rapid, irregular heart rhythms), and to restore a normal heart rhythm in paroxysmal supraventricular tachycardia (PSVT) — sudden episodes of a very fast but regular heartbeat originating above the heart's lower chambers.

Most-Reported Side Effects for Diltiazem

From FAERS, the FDA’s public side-effect reporting system (34,400 reports, all brands). Reports are voluntary and are not proof the drug caused the effect.

Dyspnea 3,309 reports
Fatigue 2,733 reports
Nausea 2,454 reports
Dizziness 2,174 reports
Diarrhea 2,113 reports
See full Diltiazem drug information
Keep checking

Other Drugs & Phenylalanine

Browse every other medication we’ve checked against Phenylalanine — both the ones with a known interaction and the ones without. Select any drug for the full detail.

5Major
11Moderate
0Minor
2,813No known interaction
5 Major 11 Moderate 2,813 No interaction
Major

5 interactions

Moderate

11 interactions

No interaction 2,813 drugs checked — show the list

We also checked Phenylalanine against these medications and found no known interaction in our sources. Open any page to confirm.

Showing the 60 most looked-up of 2,813 medications checked with no known interaction — see the full Phenylalanine list.

See the full Phenylalanine interactions page
See every supplement checked against Diltiazem, ranked by severity
Evidence & sources

Phenylalanine References

The 23 references that drive our Phenylalanine monograph and interaction data, from the evidence-graded Natural Medicines (TRC Healthcare) database. Citations with a link open the study on PubMed or the publisher’s site.

  1. Rouse B, Azen C, Koch R, et al. Maternal phenylketonuria collaborative Study (MPKUCS) offspring: facial anomalies, malformations, and early neurological sequelae. Am J Med Genet 1997;69:89-95. DOI
  2. Sturtevant FM. Use of aspartame in pregnancy. Int J Fertil 1985;30:85-7.
  3. Silkaitis RP, Mosnaim AD. Pathways linking L-phenylalanine and 2-phenylethylamine with p-tyramine in rabbit brain. Brain Res 1976;114:105-15.
  4. Lehmann WD, Theobald N, Fischer R, Heinrich HC. Stereospecificity of phenylalanine plasma kinetics and hydroxylation in man following oral application of a stable isotope-labelled pseudo-racemic mixture of L- and D-phenylalanine. Clin Chim Acta 1983;128 PubMed
  5. Mosnik DM, Spring B, Rogers K, Baruah S. Tardive dyskinesia exacerbated after ingestion of phenylalanine by schizophrenic patients. Neuropsychopharmacology 1997;16:136-46. PubMed
  6. Siddiqui AH, Stolk LM, Bhaggoe R, et al. L-phenylalanine and UVA irradiation in the treatment of vitiligo. Dermatology 1994;88:215-8. PubMed
  7. Birkmayer W, Riederer P, Linauer W, Knoll J. L-deprenyl plus L-phenylalanine in the treatment of depression. J Neural Transm 1984;59:81-7. PubMed
  8. Nutt JG, Woodward WR, Hammerstad JP, et al. The "on-off" phenomenon in Parkinson's disease. Relation to levodopa absorption and transport. N Engl J Med 1984;310:483-8. PubMed
  9. Baruzzi A, Contin M, Riva R, et al. Influence of meal ingestion time on pharmacokinetics of orally administered levodopa in parkinsonian patients. Clin Neuropharmacol 1987;10:527-37. PubMed
  10. Juncos JL, Fabbrini G, Mouradian MM, et al. Dietary influences on the antiparkinsonian response to levodopa. Arch Neurol 1987;44:1003-5. PubMed
  11. Eriksson T, Granerus AK, Linde A, et al. "On-off" phenomenon in Parkinson's disease: relationship between dopa and other large neutral amino acids in plasma. Neurology 1988;38:1245-8. PubMed
  12. Baker GB, Bornstein RA, Rouget AC, et al. Phenylethylaminergic mechanisms in attention-deficit disorder. Biol Psychiatry 1991;29:15-22.. PubMed
  13. Wood DR, Reimherr FW, Wender PH. Treatment of attention deficit disorder with DL-phenylalanine. Psychiatry Res 1985;16:21-6.. PubMed
  14. Food and Nutrition Board, Institute of Medicine. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (Macronutrients). Washington, DC: National Academy Press, 2002. Available at: http://www.n
  15. Cederbaum S. Phenylketonuria: an update. Curr Opin Pediatr 2002;14:702-6. PubMed

Medication overviews come from MedlinePlus (NIH); supplement label data from the NIH Dietary Supplement Label Database. Content is written and reviewed by licensed HelloPharmacist pharmacists — see our data sources and editorial standards.

Pharmacist Counseling Corner

Taking Phenylalanine and Diltiazem Together: Common Questions

Is it safe to take Phenylalanine with Diltiazem?
We do not list a known interaction between Phenylalanine and Diltiazem. That does not guarantee the combination is risk-free, so confirm with your pharmacist or doctor — especially if you take other medications or have a health condition.
Do Phenylalanine and Diltiazem have any of the same side effects?
Yes — our sources link each of them separately to 8 of the same effects. That is not an interaction: neither changes how your body handles the other. It does mean that if one of those effects turns up, it can be harder to tell which one is behind it. They are listed on this page with the source behind each.
How was the Phenylalanine and Diltiazem combination checked?
Interactions in our licensed database are recorded against drug classes. Diltiazem is filed under 9 classifications, and Phenylalanine has documented interactions with 3 drug classes — none of Diltiazem's classifications are among them. A medication can belong to classes beyond those recorded in our dataset, so this describes how the check was run rather than guaranteeing safety.
Does Phenylalanine interact with any other medications?
Yes. Phenylalanine has 16 documented drug interactions in our database — Diltiazem is simply not one of them. If you take anything else, it is worth checking each one; they are all listed on this page.
Does Diltiazem interact with any supplements?
Yes. Diltiazem has documented interactions with 260 supplements in our database — Phenylalanine is not one of them. The Diltiazem interaction page linked on this page lists every one, ranked by severity.

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

Where next

Keep Exploring

Phenylalanine monograph Uses, safety profile, and every drug interaction in one place Diltiazem drug information The full plain-English guide to this medication Diltiazem supplement interactions Every supplement checked against Diltiazem, ranked by severity Supplement interaction checker Check any supplement or product against your medications Browse medications A–Z Every drug we’ve checked against herbs & supplements References The 23 citations behind this page Ask a pharmacist Free, personal answer from our licensed team

Still Have a Question About Diltiazem and Phenylalanine?

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This information is for education only and is not a substitute for professional medical advice. Always check with your pharmacist or doctor before starting, stopping, or combining supplements and medications.