Supplements and Co-trimoxazole.
Co-trimoxazole, sold under the brand names Septrin, Bactrim, Septra, is classified under "sulfonamide trimethoprim" in the BNF.
Co-trimoxazole is an antibiotic, usually taken as a short course, and the supplement questions people ask most when given one have reassuring answers. Probiotics are fine to take alongside it and lower the chance of antibiotic-related diarrhoea: space a bacterial probiotic, most Lactobacillus or Bifidobacterium products, about two hours from your dose, while Saccharomyces boulardii, which is a yeast, can be taken at the same time. Most everyday vitamins and minerals have no significant interaction with a short course of Co-trimoxazole, so a daily multivitamin, vitamin D or vitamin C is generally not a concern, with one exception that matters: Co-trimoxazole makes the body hold on to potassium, so do not add a potassium supplement, a potassium-based salt substitute or a high-potassium multi-mineral product while you are taking it unless your doctor is monitoring your blood potassium. The combinations that genuinely matter are listed below with their evidence, and they tend to be specific: supplements that thin the blood, serotonergic supplements stacked on certain antibiotics, and supplements that are really a prescription drug in disguise, such as red yeast rice, which contains the statin lovastatin. The sensible step in every case is to tell your pharmacist what you already take, which is free at any UK community pharmacy.
Below are the 3 documented pairs we have explicitly assessed against Co-trimoxazole in the Distil database: 1 red and 2 green. The pairs cluster around 2 mechanisms: Additive hyperkalaemia (raised potassium) and Probiotic timing with antibiotics. Every call is cited to either a clinical reference (PMID) or the British National Formulary. Anything not on this list is either still to be assessed or beyond our database scope. The checker beneath surfaces assessments by supplement, and the missing-item form at the bottom of the page routes any uncatalogued supplement into our next curation pass.
Documented interactions
Last reviewed 15 June 2026. Each interaction below lists the evidence its assessment drew on: some rest on studies of the exact pair, others on mechanism evidence, where a citation may support one part of the reasoning rather than the exact combination. The evidence line on each entry says which.
Additive hyperkalaemia (raised potassium)
Red Does Potassium interact with ?
Co-trimoxazole contains trimethoprim, which makes the body hold on to potassium by acting on the kidney in much the same way as the potassium-sparing diuretic amiloride. Adding a potassium supplement on top can push potassium to a dangerous level and affect the heart rhythm, and in older people on certain heart or blood-pressure medicines this combination has been linked to sudden death. Do not take potassium with co-trimoxazole unless your specialist has advised it and is monitoring your blood potassium closely.
Evidence: Well established across multiple human studies or interaction references.
Probiotic timing with antibiotics
Green Does Bifidobacterium longum 1714 interact with ?
Bifidobacterium longum 1714 and co-trimoxazole are fine to use together. Bifidobacterium longum 1714 is a live bacterium, so space it about two hours away from your co-trimoxazole dose so the antibiotic does not reduce the live bacteria. The main exception is if your immune system is seriously weakened or you are critically ill in hospital, when any probiotic should be checked with your doctor first.
Evidence: Supported by human study data.
Green Does Probiotics interact with ?
Probiotics and co-trimoxazole are fine to use together, and probiotics may lower the chance of antibiotic-related diarrhoea. If you take a bacterial probiotic (most Lactobacillus or Bifidobacterium products), space it about two hours away from your co-trimoxazole dose so the antibiotic does not reduce the live bacteria. Saccharomyces boulardii is a yeast rather than a bacterium, so the antibiotic does not affect it and it can be taken at the same time. The main exception is if your immune system is seriously weakened or you are critically ill in hospital, when probiotics, particularly Saccharomyces boulardii, should be checked with your doctor first.
Evidence: Supported by human study data.
What this list does not say. Pairs not flagged here are not implicitly safe. They are either not yet in our database, or fall outside our inclusion scope (food-supplement interactions only; for drug-drug interactions, the BNF is authoritative). Use the checker below to surface any supplement, and submit a missing item if you take something we have not catalogued.
No affiliates. No commission. We earn nothing on what you buy.
How we grade severity, choose what's in scope, and what we exclude.
Every call on this page is reasoned. We publish the full rubric for severity tiers, the medication inclusion logic, the evidence grades we accept, and what we deliberately leave out. About three thousand words. Worth reading once if you use this tool more than occasionally.
Read the full methodologyA full supplement review, built around you, not a generic stack.
The free checker answers one interaction question at a time. A Distil report reviews the bigger picture: your goals, diet, medications, current supplements and any blood results you provide, then shows what appears worth keeping, changing, adding or leaving out.
- what your diet already covers, so you are not adding a capsule for something food is already handling
- which evidence-graded compounds are worth considering for your goals, and which were considered but left out
- what to keep, change or stop from what you already take, plus the order to introduce anything new
- how your medications and complete supplement stack fit together, with daily timing and a clear point for reassessment
The interaction check is one section of the full report. You also get the reasoning behind each recommendation and the evidence references used.
£49 founder pricing while Distil is new, or returning within 6 months · £79 standard.
Something missing?
If a supplement or medication you take isn't in our autocomplete, tell us. We go through what people flag every week and add what's missing.