Supplements and Trimethoprim.
Trimethoprim, sold under the brand names Monotrim, Primsol, is classified under "folate antagonist antibacterial" in the BNF.
Trimethoprim 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 antibiotic course, so a daily multivitamin, vitamin D or vitamin C is generally not a concern. 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 Trimethoprim 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 is assessed against PubMed-indexed evidence.
Additive hyperkalaemia (raised potassium)
Red Does Potassium interact with ?
Trimethoprim 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. This is one of the higher-risk combinations. Do not take potassium with trimethoprim unless your specialist has advised it and is monitoring your blood potassium closely, especially if you are older or have kidney problems.
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 trimethoprim are fine to use together, and the strain may lower the chance of antibiotic-related diarrhoea. Bifidobacterium longum 1714 is a live bacterium, so space it about two hours away from your trimethoprim 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 trimethoprim 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 trimethoprim 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.
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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.
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This page checks the pairs you enter. The personalised Distil report goes further:
- the same graded, cited interaction check across your whole stack, not just the pairs you thought to type in
- where your current routine may be leaving you short of your goals
- the evidence-graded compounds worth adding, and the ones worth dropping
It's a paid report: £79, or £49 founder pricing while we earn our first reviews. The interactions check is one section of it, and you can read a real one in full before you decide.
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