Drug & supplement interaction

Soy + Sulfamethoxazole

Sulfamethoxazole brand name: Gantanol.

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

Sulfamethoxazole falls into 2 drug categories that Soy can interact with; the most serious is rated minor. Theoretically, antibiotics may decrease the activity of soy isoflavones.

Read the full interaction report
SeverityMinor
LikelihoodPossible
Interactions2
EvidenceD
What do Severity, Likelihood & Evidence mean?
Know someone taking this? Share
Facebook Post Reddit Email
On this page
Interaction report

Soy and Sulfamethoxazole

These two interact in 2 distinct ways. Each one is explained separately below.

Why Are There 2 Interactions?

Soy interacts with whole categories of medication. Sulfamethoxazole falls into 2 of those categories, so it can interact with Soy in 2 different ways. We’ve listed each one below — your overall risk is driven by the most serious of them (Minor).

Minor

ANTIBIOTIC DRUGS

1 of 2

Why this applies to Sulfamethoxazole

Sulfamethoxazole falls under ANTIBIOTIC DRUGS — the group Soy interacts with.

Theoretically, antibiotics may decrease the activity of soy isoflavones.
Intestinal bacteria are responsible in part for converting soy isoflavones into their active forms. Antibiotics may decrease the amount of intestinal bacteria and decrease its ability to convert isoflavones.
Severity Minor Likelihood Possible Evidence D

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.

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

Here's the thing to know about this pairing, and it's actually pretty mild. Taking sulfamethoxazole (often combined with trimethoprim as Bactrim) doesn't get changed by soy. It's the other way around: this interaction is about soy possibly working a little less well, not your antibiotic.

Your gut bacteria help turn soy isoflavones into their more active forms. Since antibiotics like sulfamethoxazole can knock down some of those bacteria, the soy you eat might be slightly less active for a short while. This is only theoretical, based on lab work, and it's rated minor. Eating tofu, edamame, or soy milk during a normal antibiotic course is unlikely to cause any real problem. If you take concentrated soy isoflavone supplements, just check with your pharmacist.

Contact Dr. Brian directly if you have additional questions on this interaction
References
  1. Morito K, Hirose T, Kinjo J, et al. Interaction of phytoestrogens with estrogen receptors alpha and beta. Biol Pharm Bull 2001;24:351-6. PubMed
Minor

CYTOCHROME P450 2C9 (CYP2C9) SUBSTRATES

2 of 2

Sulfamethoxazole also falls under CYTOCHROME P450 2C9 (CYP2C9) SUBSTRATES.

Soy might modestly induce CYP2C9 enzymes. However, this effect does not seem to be clinically significant.
In vitro research suggests that an unhydrolyzed soy extract might induce CYP2C9. However, the significance of this interaction is likely minimal. In healthy females taking a specific extract of soy (Genistein Soy Complex, Source Naturals), blood levels of losartan, a CYP2C9 substrate, were not significantly affected.
Severity Minor Likelihood Unlikely Evidence D

Interaction has been demonstrated in animal or in lab research but has been shown not to occur in humans..

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

Here's the short version: soy and sulfamethoxazole (Gantanol) really don't have much of a meaningful interaction to worry about. The theory is that soy might slightly speed up a liver enzyme called CYP2C9, which is one of the routes your body uses to process sulfamethoxazole. In plain terms, that could in theory clear the antibiotic a touch faster and make it a bit weaker, but the evidence here is very thin.

This comes only from lab and test-tube studies, and a human study with a similar CYP2C9 drug showed no real change. Enjoying soy foods in your diet should not affect how well your antibiotic works. If you're taking concentrated soy isoflavone supplements, just mention it to your pharmacist or doctor so they can confirm you're in the clear.

Contact Dr. Brian directly if you have additional questions on this interaction
References
  1. Wang G, Xiao CQ, Li Z, et al. Effect of soy extract administration on losartan pharmacokinetics in healthy female volunteers. Ann Pharmacother 2009;43:1045-9. PubMed

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

Soy Also Interacts With Other Medications

Soy has 9 major, 343 moderate, 258 minor known drug interactions in our database — the Sulfamethoxazole one above is among them. If you take anything else, check it against the full list below.

Major 9 Moderate 343 Minor 258 No known interaction 2,220 (incl. Sulfamethoxazole)
Taking anything else?

Check Another of Your Medications Against Soy

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

Everything else you take

See Every Supplement That Interacts With Sulfamethoxazole

This page covers Soy only. If you take other supplements — a multivitamin, a sleep aid, something for joints — the Sulfamethoxazole page lists 162 supplements it has a documented interaction with, worst first, each one explained.

The most serious there right now: Activated Charcoal (major), Beer (major), Para-aminobenzoic Acid (paba) (major), Siberian Cocklebur (major) and Wine (major).

By severity: 5 major · 134 moderate · 23 minor.

From our pharmacist’s overview on the Sulfamethoxazole page

“Sulfamethoxazole (Gantanol) is a sulfa antibiotic, and a few supplements can genuinely undermine it or harm you while taking it, so this is a list worth reading closely. The supplement PABA can directly block how the antibiotic kills bacteria, and it is one of five major-rated entries among the 162 on file.”

Reviewed by 2 · Jul 18, 2026

Open the Sulfamethoxazole interaction page

One more smart check

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

We hold 821 real supplement products containing Soy, each one broken down to its full ingredient list — and combination products often carry several ingredients besides Soy. Your product may carry other ingredients that interact with Sulfamethoxazole too, on top of Soy — 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 Soy and Sulfamethoxazole 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 Soy
Top 25%

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

29 days ago today
Medication Sulfamethoxazole
Top half

#910 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 Soy

What is Soy?

  • Menopause symptoms (hot flashes)
  • Heart and cholesterol health
  • Bone health
  • Plant-based protein source
  • General nutrition

Soy is a nutritious bean that is a staple food and a popular source of plant protein and isoflavones. Eating soy foods as part of a balanced diet is generally considered safe for most people, and there is moderate evidence it may modestly help cholesterol and some menopause symptoms, though results vary. Talk with your pharmacist or doctor before using concentrated soy isoflavone supplements, especially if you have a history of hormone-sensitive conditions.

Read the full Soy monograph & all interactions
Keep checking

Other Drugs & Soy

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

9Major
343Moderate
258Minor
2,219No known interaction
9 Major 343 Moderate 258 Minor 2,219 No interaction
Major

9 interactions

Moderate

343 interactions

Showing 60 of 343 moderate interactions — see all 343 on the Soy page.

Minor

258 interactions

Showing 60 of 258 minor interactions — see all 258 on the Soy page.

No interaction 2,219 drugs checked — show the list

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

Showing the 60 most looked-up of 2,219 medications checked with no known interaction — see the full Soy list.

See the full Soy interactions page
See every supplement checked against Sulfamethoxazole, 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
Soy monograph › Uses, safety, and every drug interaction in one place Sulfamethoxazole supplement interactions › Every supplement checked against Sulfamethoxazole, 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 Ask a pharmacist › Free, personal answer from our licensed team
Pharmacist Counseling Corner

Taking Soy and Sulfamethoxazole Together: Common Questions

Can you take Soy with Sulfamethoxazole?
There is only a minor interaction, which is usually not serious. Read the full details on this page and confirm with your pharmacist before combining them.
Why are there 2 interactions listed for Soy and Sulfamethoxazole?
Sulfamethoxazole belongs to more than one drug category, and Soy interacts with each of those categories a little differently. We list every one so nothing is missed — your overall risk is driven by the most serious of them.
How serious is the Soy and Sulfamethoxazole interaction?
We rate it Minor. This interaction is generally not clinically significant, but it remains worth noting and discussing with your pharmacist.

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

Still Have a Question About Sulfamethoxazole and Soy?

Our pharmacists answer your medication & supplement questions — free.

Ask a pharmacist

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.