Herb & supplement monograph

Onion Drug Interactions, Uses, Effectiveness, Safety & More

Medically reviewed by Dr. Brian Staiger, PharmD, BCPS ·Updated July 2026

What is this page for?

First and foremost: how Onion interacts with medications. The heart of this page is the interaction list — every drug Onion is known to interact with, and how serious each one is.

But these pages have grown well beyond that into a full monograph — what Onion is, what people use it for and how strong the evidence is, its safety and side effects, and answers to the questions we’re asked most — written and reviewed by the clinical staff at HelloPharmacist. It’s educational information from our licensed clinical databases, not medical advice, and we don’t sell or endorse products. Our editorial policy

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Onion Drug Interactions: The Bottom Line

From the HelloPharmacist Editorial Team · Updated July 2026

Based on the available evidence, the overall risk of a clinically significant Onion drug interaction appears low for most people. None of the listed interactions are rated major, and most rest on lab or animal findings rather than effects proven in people.

Interactions deserving the most attention

A few combinations have the best human support and deserve the most care. Onion may act as a mild diuretic, which could reduce how the body clears lithium and raise its levels. In one report, aspirin worsened an existing onion allergy, causing swelling and severe hives, so this matters mainly for people already allergic to onion. Onion has also been shown to lower blood sugar in studies, so people on diabetes medications may see their glucose run lower than expected.

What the rest of the list means

Most of the long list of medications comes from one theoretical concern: animal research suggests onion may slow an enzyme the body uses to clear certain medications, but this has not been reported in humans. Similarly, onion affected platelets in lab studies, yet an added bleeding risk with blood thinners has not been confirmed in people. So a long list does not mean every combination is truly risky.

Check your medications against Onion

Based on HelloPharmacist’s Onion interaction data and reviewed under our editorial standards.

Interaction report

Drugs that interact with Onion

275 medications have a known interaction with Onion, graded by severity. Select any drug for the full evidence-based detail.

Don’t want to scroll the list? Just ask.

Tell us the medications you take and we’ll check each one against Onion using our pharmacist-reviewed interaction data.

AI summaries are generated from our Onion interaction data for education only — always confirm with your pharmacist. How we use AI

2,830 drugs

Read The List Like a Pharmacist

What Severity, Likelihood & Evidence Mean

Severity — How Serious It Can Be

Major. Clinically significant; generally best avoided, or used only under direct professional supervision.
Moderate. May need monitoring, a dose adjustment, or separating the times you take each one.
Minor. Generally not clinically significant, but still worth noting and mentioning to your pharmacist.
No known interaction. Checked against our sources with nothing documented — not the same as proven safety.

Likelihood — How Well It’s Documented

Likely. Well-controlled human studies have demonstrated the likely existence of this interaction
Probable. 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.
Possible. Interaction 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
Unlikely. Interaction has been demonstrated in animal or in lab research but has been shown not to occur in humans.

Where This Data Comes From

Interaction records are evidence-graded and sourced from the Natural Medicines database (TRC Healthcare), the same reference used by pharmacists and hospitals.
Each drug listed above links to the full report for that exact Onion combination — clinical detail, likelihood, evidence level, and citations.
Content is reviewed by licensed HelloPharmacist pharmacists — see our data sources and editorial standards.

The big picture

The kinds of drugs Onion affects

Every type of medication (drug category) Onion is known to interact with. Open any category for the detail — or search your exact drug in the checker above.

All 5 drug categories Onion interacts with

Anticoagulant/antiplatelet DrugsAntidiabetes DrugsAspirinCytochrome P450 2e1 (cyp2e1) SubstratesLithium

Anticoagulant/Antiplatelet Drugs

Theoretically, concomitant use of anticoagulant or antiplatelet drugs with onion might increase the risk of bleeding.
In vitro research shows that onion inhibits platelet aggregation.

Likelihood Possible Evidence D

Antidiabetes Drugs

Concomitant use of antidiabetes drugs with onion may increase the risk of hypoglycemia.
Animal research and clinical research shows that taking onion can lower blood glucose levels. Monitor blood glucose levels closely.

Likelihood Possible Evidence B

Aspirin

Concomitant use of aspirin with onion may worsen onion allergy.
In one case report, a patient with a mild onion allergy reported worsening allergy, including swelling and severe urticaria, after taking aspirin.

Likelihood Probable Evidence D

Cytochrome P450 2E1 (Cyp2E1) Substrates

Theoretically, taking onion might increase the levels and clinical effects of drugs metabolized by CYP2E1.
Animal research shows that taking onion powder inhibits CYP2E1. However, this interaction has not been reported in humans.

Likelihood Possible Evidence B

Lithium

Onion is thought to have diuretic properties. Theoretically, due to these potential diuretic effects, onion might reduce excretion and increase levels of lithium. The dose of lithium might need to be decreased.

Likelihood Probable Evidence D

Monograph

Onion: Uses, Safety & Side Effects

Medically reviewed by Dr. Brian Staiger, PharmD, BCPS · Reviewed July 2026 ·

The bottom line

Onion is a common food with a long history in traditional medicine, and it contains antioxidants like quercetin and sulfur compounds. Eating onion as part of a balanced diet is safe for most people, but high-dose supplements have only limited human evidence for any specific health condition. Talk with your pharmacist or doctor before using concentrated onion products for a medical reason.

Scientific name: Allium cepa
Family: Amaryllidaceae
Part used: Bulb (and sometimes the green leaves and extracted juice)
Common forms: Fresh bulb, juice, dried powder, capsules, extracts, and topical gels

People commonly use it for

Colds and respiratory symptoms
Heart and cholesterol health
Blood sugar support
Antimicrobial and wound care
Scar appearance (topical gels)

Popular and traditional uses — not proof it works. See “Uses & effectiveness” below for the evidence.

Safety at a glance

OverallLikely safe

Onion as a food is generally safe for most people; concentrated supplements are less studied.

PregnancyInsufficient reliable information

Insufficient reliable information available; avoid using in amounts greater than used in foods.

Read the full pregnancy detail

BreastfeedingInsufficient reliable information

Insufficient reliable information available; avoid using in amounts greater than used in foods.

Read the full breastfeeding detail

Pregnancy & breastfeeding ratings are from Natural Medicines (Therapeutic Research Center). Safety guidance is general; always confirm with your pharmacist or doctor for your situation.

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Overview

Onion (Allium cepa) is one of the most widely eaten vegetables in the world. It is a bulb plant in the same family as garlic, leeks, and chives, and it is grown in nearly every country.

People have used onion in cooking and folk medicine for thousands of years. The bulb, its juice, and sometimes the green tops are used. Beyond food, onion is found in supplements, syrups, and topical gels. Many supplements are standardized to certain compounds, such as quercetin or sulfur-based ingredients.

How it works

Onion contains several compounds thought to be active, including quercetin (an antioxidant flavonoid) and various sulfur compounds that give onion its sharp smell and taste. It also provides fiber and prebiotic substances that may support gut bacteria.

In laboratory and animal studies, these compounds appear to have antioxidant, anti-inflammatory, and antimicrobial effects. Some research suggests they may relax blood vessels, affect cholesterol, or influence blood sugar. Keep in mind that most of this evidence comes from test-tube and animal research, and effects in people may be much smaller or different.

Effectiveness

Does Onion work?

Here’s what the research actually shows for each use — graded by Natural Medicines and reviewed by Dr. Brian Staiger, PharmD.

Evidence overview · 19 uses evaluated

1 Possibly effective 1 Leans ineffective 17 Insufficient evidence

How to read these evidence grades

Natural Medicines’ 7-point scale. We show each rating’s label word-for-word.

1EffectiveStrong, consistent evidence it works.

2Likely EffectiveGood evidence, though not yet conclusive.

3Possibly EffectiveSome evidence suggests a benefit.

4Possibly IneffectiveSome evidence it may not help.

5Likely IneffectiveFairly strong evidence it doesn’t help.

6IneffectiveStrong evidence it doesn’t work.

7Insufficient Reliable Evidence to RateToo little research to say either way.

Possibly Effective Scarring