Drug & supplement interaction

Fish Oil + Orlistat

Orlistat brand names: Xenical, Alli.

Moderate Interaction
Medically reviewed by · Updated September 2026
The bottom line Moderate

Fish Oil has a moderate interaction with Orlistat. Theoretically, taking fish oil with orlistat might decrease the absorption of fish oil fatty acids.

Read the full interaction report
SeverityModerate
LikelihoodProbable
EvidenceD
What do Severity, Likelihood & Evidence mean?
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Interaction report

Fish Oil and Orlistat

Here is the interaction we found between these two, with the evidence behind it.

Moderate

ORLISTAT (Xenical, Alli)

Theoretically, taking fish oil with orlistat might decrease the absorption of fish oil fatty acids.
Orlistat binds lipase in the gastrointestinal tract and reduces fat absorption. Theoretically, taking fish oil with orlistat might decrease absorption of fish oil fatty acids. To avoid this potential interaction, recommend separating administration of orlistat and fish oil by at least 2 hours.
Severity Moderate Likelihood Probable Evidence D

Interaction has not been documented in well-controlled studies, however, the interaction has been demonstrated in some small human studies or in controlled animal studies in conjunction with multiple case reports..

Dr. Brian Staiger, PharmD, BCPS
Dr. Brian Staiger explains PharmD, Clinical Pharmacist

Here's the simple version: orlistat works right in your gut, where it blocks the enzyme (lipase) that breaks down dietary fat so you absorb less of it. Since the omega-3 fats in fish oil (EPA and DHA) are also fats, orlistat may keep some of those from being absorbed too. So this isn't about orlistat building up or weakening. It's about your fish oil possibly working a little less well.

One thing to keep in mind: this concern is theoretical, based on how the drug works rather than hard studies, so it may be a small effect. An easy fix is timing. Taking your fish oil and orlistat at least 2 hours apart should help. If you're using fish oil for a real reason like high triglycerides, it's worth running this past your pharmacist or doctor first.

Contact Dr. Brian directly if you have additional questions on this interaction

What These Ratings Mean

Every interaction above is graded three ways — here’s the key to each label.

Severity how clinically significant the interaction is
Major

Clinically significant. Avoid the combination, or use only under close professional supervision.

Moderate

Usually worth avoiding or managing — may need monitoring, a dose change, or timing apart.

Minor

Generally not serious. Be aware of it and mention it to your pharmacist.

Likelihood how strong the evidence is that the interaction actually happens in people
  • LikelyWell-controlled human studies have demonstrated the likely existence of this interaction.
  • ProbableInteraction has not been documented in well-controlled studies, however, the interaction has been demonstrated in some small human studies or in controlled animal studies in conjunction with multiple case reports.
  • PossibleInteraction has been documented in animal or in lab research, or the interaction has been documented in humans but is limited to case reports or conflicting clinical research exists.
  • UnlikelyInteraction has been demonstrated in animal or in lab research but has been shown not to occur in humans.
Evidence the quality of the studies behind the rating (graded A–D)
  • AHigh-quality meta-analysis (quantitative systematic review).
  • BCase-control study.
  • CConsensus.
  • DAnecdotal evidence.

These ratings are for information only. Never start, stop, or otherwise change your therapy before speaking with your provider.

Keep it in perspective

Fish Oil Also Interacts With Other Medications

Fish Oil has 206 moderate, 120 minor known drug interactions in our database — the Orlistat one above is one of them. If you take anything else, check it against the full list below.

Moderate 206 Minor 120 No known interaction 2,504 (incl. Orlistat)
Taking anything else?

Check Another of Your Medications Against Fish Oil

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

From our Q&A

Questions About Fish Oil or Orlistat

These are real questions from readers, answered by the pharmacists at HelloPharmacist — written by a person, not generated from a database. Each one covers Fish Oil or Orlistat, 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 Orlistat

This page covers Fish Oil only. If you take other supplements — a multivitamin, a sleep aid, something for joints — the Orlistat page lists 18 supplements it has a documented interaction with, worst first, each one explained.

The most serious there right now: Activated Charcoal (major), Coffee Charcoal (moderate), European Mandrake (moderate), Glucomannan (moderate) and Guar Gum (moderate).

By severity: 1 major · 8 moderate · 9 minor.

From our pharmacist’s overview on the Orlistat page

“People who take Orlistat (Xenical, Alli) for weight loss often browse the very supplement aisle where fiber products and fish oil sit, and both show up on this page, so it deserves genuine attention before you mix anything in. Nearly all 18 ingredients on file can change how oral medications absorb from the gut, and the single major-ra…”

Reviewed by 2 · Jul 18, 2026

Open the Orlistat interaction page

One more smart check

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

We hold 4,777 real supplement products containing Fish Oil, each one broken down to its full ingredient list — and combination products often carry several ingredients besides Fish Oil. Your product may carry other ingredients that interact with Orlistat too, on top of Fish Oil — and those would not show on this page. Find your product and we’ll check every ingredient on its label:

How much people look this up

Where Fish Oil and Orlistat 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 Fish Oil
Top 5%

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

29 days ago today
Medication Orlistat
Top half

#1,041 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.

Supplement

About Fish Oil

What is Fish Oil?

Fish Oil
  • High triglycerides
  • Heart health
  • Joint pain and inflammation
  • Brain and eye health
  • Dry eyes

Fish oil provides omega-3 fatty acids (EPA and DHA) that are best known for lowering high triglyceride levels. The evidence for other heart and health benefits is mixed, and it is generally well tolerated. Talk with your pharmacist or doctor before starting it, especially if you take blood thinners or have a fish allergy.

Read the full Fish Oil monograph & all interactions
Medication

About Orlistat

What Is Orlistat Used For?

Brand names: Xenical, Alli

Xenical and prescription-strength orlistat are approved to help obese adults lose weight and keep it off when used with a reduced-calorie diet. They are intended for people with a body mass index (BMI) of 30 or higher, or 27 or higher if other risk factors such as high blood pressure, diabetes, or dyslipidemia (abnormal cholesterol or fat levels in the blood) are present.

Alli is the over-the-counter version, approved for weight loss in overweight adults 18 and older when used with a reduced-calorie, low-fat diet and exercise program.

See full Orlistat drug information
Keep checking

Other Drugs & Fish Oil

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

0Major
206Moderate
120Minor
2,503No known interaction
206 Moderate 120 Minor 2,503 No interaction
Moderate

206 interactions

Showing 60 of 206 moderate interactions — see all 206 on the Fish Oil page.

Minor

120 interactions

Showing 60 of 120 minor interactions — see all 120 on the Fish Oil page.

No interaction 2,503 drugs checked — show the list

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

Showing the 60 most looked-up of 2,503 medications checked with no known interaction — see the full Fish Oil list.

See the full Fish Oil interactions page
See every supplement checked against Orlistat, ranked by severity
Sources

Sources & How We Checked

Interaction details are evidence-graded and sourced from the Natural Medicines database. The medication overview comes from MedlinePlus.

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

Parts of this content are provided by the Therapeutic Research Center, LLC.

DISCLAIMER: Currently this does not check for drug-drug interactions. This is not an all-inclusive comprehensive list of potential interactions and is for informational purposes only. Not all interactions are known or well-reported in the scientific literature, and new interactions are continually being reported. Input is needed from a qualified healthcare provider including a pharmacist before starting any therapy. Application of clinical judgment is necessary.

© 2021 Therapeutic Research Center, LLC

Keep exploring
Fish Oil monograph › Uses, safety, and every drug interaction in one place Orlistat supplement interactions › Every supplement checked against Orlistat, ranked by severity Orlistat drug information › The full plain-English guide to this medication Supplement interaction checker › Check any supplement or product against your medications Browse medications A–Z › Every drug we've checked against herbs & supplements Ask a pharmacist › Free, personal answer from our licensed team
Pharmacist Counseling Corner

Taking Fish Oil and Orlistat Together: Common Questions

Can you take Fish Oil with Orlistat?
There is a moderate interaction, so it may be acceptable with monitoring or timing changes — but check with your pharmacist first. Read the full details on this page and confirm with your pharmacist before combining them.
How serious is the Fish Oil and Orlistat interaction?
We rate it Moderate. This combination may require monitoring, a dose adjustment, or separating the times you take each one. Consult your pharmacist before using them together.

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

Still Have a Question About Orlistat and Fish Oil?

Our pharmacists answer your medication & supplement questions — free.

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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.