Co-trimoxazole: The Antibiotic With the Worst Interactions
Most antibiotics are prescribed, taken and forgotten. This one deserves a different level of attention, because it interacts with a longer list of ordinary medicines than almost anything else in the cupboard, and several of those combinations have killed people who were taking both for entirely sensible reasons.
None of that makes it a bad drug. It is effective, inexpensive, and for certain infections it is the treatment of choice with no close rival. It simply requires that somebody checks the rest of the list first, which is exactly what does not happen when a short course is issued quickly for a urine infection.
⚗️ Two drugs, and the reason they are combined. Bacteria have to manufacture folate themselves, and they use a two-step assembly line to do it. One component of this tablet blocks the first step and the other blocks the second, so the blockage is far more complete than either alone achieves.
Human cells do not use that pathway — we absorb folate ready-made from food, which is why the drug is selective. But it does interfere with folate handling in some circumstances, and almost every serious problem below traces back to that: the interaction with a drug that also blocks folate, the caution in pregnancy, and the effect on blood cell production.
🎯 What it is actually used for
The name is best known in connection with urine infections, and in much of the world that is no longer its main role.
Resistance changed the picture. In many regions a substantial share of the bacteria causing simple urine infections are now resistant to it, which is why several countries moved it off first-line for that indication in favour of alternatives such as nitrofurantoin. Where local resistance is low it remains a good choice; where it is high, prescribing it blind means a course that does nothing while the infection continues.
Meanwhile it holds a set of roles where it has no real competitor: preventing and treating a serious lung infection in people whose immune systems are suppressed, treating certain resistant skin and soft tissue infections, and treating several uncommon organisms. In those settings it is not one option among many — it is the drug.
Which means a person may be taking it for weeks or months rather than for three days, and the interaction list matters far more in that situation.
🚨 The combinations that cause harm
🔴 Tell whoever prescribes this every single medicine you take, including ones you have taken for years without incident. These are the ones that matter most.
Warfarin. This antibiotic substantially raises the effect of warfarin, and the result is major bleeding. It is among the most dangerous of all antibiotic interactions with warfarin, and it develops over days rather than immediately. Where the combination is unavoidable, monitoring has to be increased and the warfarin dose usually reduced in advance.
Methotrexate. Both interfere with folate, and together they can cause severe suppression of the bone marrow — a collapse in white cells, red cells and platelets that has proved fatal. This combination should generally be avoided outright rather than monitored.
Blood pressure medicines ending in pril or sartan, and potassium-sparing water tablets. This antibiotic raises potassium in its own right. Added to a drug already doing the same, levels can reach the range that disturbs heart rhythm. Population studies in older patients have found a measurable excess of sudden death when this antibiotic was given rather than an alternative. The article on high potassium covers what that looks like and why it gives so little warning.
Diabetes tablets of the sulfonylurea group, such as gliclazide, glimepiride and glipizide. It raises their effect and can cause blood sugar to fall dangerously low.
Phenytoin and digoxin. Blood levels of both rise, and both have narrow margins between an effective dose and a toxic one.
The pattern worth noticing is that none of these are exotic. They are warfarin, blood pressure tablets, diabetes tablets and heart tablets — the contents of an ordinary older person medicine cabinet. That is precisely why the check matters, and why it is so often skipped for what looks like a routine three-day course.
🩸 The rash that is an emergency
Ordinary side effects are unremarkable: nausea, loss of appetite, a mild rash, headache. Taking it with food helps the first two.
One reaction is not ordinary. Drugs of this family are among the leading causes of a severe blistering skin reaction in which the surface of the skin and the linings of the mouth, eyes and genitals detach. It is rare, it is life-threatening, and it begins in a way that is easy to dismiss.
Stop the drug and seek emergency care for: a rash that is spreading rapidly, any blistering or peeling, sores or ulcers in the mouth, eyes or genitals, a rash accompanied by fever or a general sense of being seriously unwell, or skin that becomes painful and tender to touch. Flu-like symptoms preceding a rash by a day or two are a recognised warning.
A separate reaction worth knowing about is a marked fall in white blood cells, which leaves the body unable to fight infection. Sore throat, mouth ulcers, fever or unusual bruising during a course needs a blood count rather than reassurance, particularly on prolonged treatment.
🧪 The alarming blood result that is not real
This is the reverse of the section above, and it saves a good deal of unnecessary investigation.
Blood tests taken during a course frequently show a rise in creatinine, the standard marker of kidney function, and the natural conclusion is that the drug is damaging the kidneys.
Usually it is not. Trimethoprim blocks the specific transport mechanism by which the kidney secretes creatinine into urine, so the level in blood rises while the actual filtering function of the kidney is unchanged. It reverses within days of finishing the course. It is a measurement artefact rather than kidney injury.
The reason to know this is that genuine kidney injury can also occur, so the two need distinguishing rather than either being assumed. A modest rise with normal urine output and no other abnormality is usually the artefact; a larger rise, reduced urine output, or a rise in potassium alongside it is not, and needs assessment.
🤰 Pregnancy, and the timing that matters
Because this drug interferes with folate, it is generally avoided in the first trimester, where folate is what allows the structure becoming the brain and spinal cord to close properly. That structure completes at around four weeks after conception — before most women know they are pregnant, which is why the article on folic acid before conception matters here as much as anywhere.
It is also avoided close to delivery, for a different reason involving the newborn ability to handle bilirubin.
Practically: anyone who could be pregnant should say so before starting, and anyone who discovers a pregnancy while taking it should seek advice promptly rather than either panicking or ignoring it. Alternatives exist for almost every indication in which it would be used in pregnancy.
☀️ Practical points while taking it
- Take it with food and plenty of fluid — reduces nausea and reduces the small risk of crystals forming in the urine
- Expect sun sensitivity. Skin burns more easily than usual, so cover up and use protection during the course and for a short time afterwards
- Finish the course as prescribed, and go back if there is no improvement within two to three days — given resistance rates, a failure to respond is a real possibility rather than a sign of impatience
- Report a sore throat, mouth ulcers or fever during a long course
- Long courses need folic acid alongside in some situations — worth asking rather than assuming
- Mention any sulfa allergy. A previous reaction to this class is a genuine reason to avoid it, though it does not automatically rule out unrelated drugs that happen to contain sulfur
- Mention G6PD deficiency if you have it, since this drug can trigger destruction of red cells
🩺 When to seek help
- A spreading rash, blistering or peeling skin, or sores in the mouth, eyes or genitals — emergency, stop the drug
- Rash with fever, or skin that becomes painful to touch — emergency
- Sore throat, mouth ulcers, fever or unusual bruising during a course — needs a blood count
- Muscle weakness, palpitations or an irregular pulse — possible high potassium, particularly on blood pressure treatment
- Unusual bleeding or bruising on warfarin — urgent
- Symptoms of low blood sugar on diabetes tablets — shakiness, sweating, confusion
- No improvement after two to three days — the organism may be resistant
- Yellowing of skin or eyes, or dark urine
- Pregnancy discovered while taking it — seek advice promptly
- Before starting, if you take warfarin, methotrexate, a blood pressure tablet ending in pril or sartan, a sulfonylurea, phenytoin or digoxin
This antibiotic is stocked as Bactrim. For simple urine infections where resistance is a concern, Macrobid (nitrofurantoin) is the more common first choice in many places and carries none of the interactions described above. The medicines requiring particular care alongside this drug include Coumadin (warfarin), Dilantin (phenytoin), Lanoxin (digoxin), the sulfonylureas Amaryl and Glucotrol, and everything in the blood pressure category ending in pril or sartan.
❓ Frequently asked questions
Why is this two drugs in one tablet?
Because bacteria manufacture folate on a two-step assembly line, and each component blocks one step. The blockage is far more complete than either achieves alone. Human cells absorb folate ready-made from food and do not use that pathway, which is what makes the drug selective — and the residual interference with folate handling is where almost every serious problem with it originates.
Is it still a good choice for a urine infection?
It depends entirely on local resistance. In many regions a substantial share of the bacteria causing simple urine infections are now resistant, which is why several countries moved it off first-line in favour of alternatives such as nitrofurantoin. Where resistance is low it remains effective and cheap. Where it is high, a blind prescription means a course that does nothing while the infection continues.
Which combinations are genuinely dangerous?
Warfarin, where it substantially raises the anticoagulant effect and causes major bleeding. Methotrexate, where both block folate and the combination can collapse the bone marrow, sometimes fatally. Blood pressure medicines ending in pril or sartan and potassium-sparing water tablets, where potassium can reach rhythm-disturbing levels. Sulfonylurea diabetes tablets, causing dangerously low blood sugar. And phenytoin and digoxin, whose levels rise.
Why does it raise potassium?
Trimethoprim blocks a channel in the kidney through which potassium is excreted, by a mechanism resembling that of potassium-sparing diuretics. On its own in a healthy person that rarely matters. Added to a drug already reducing potassium excretion, it does — studies in older patients have found a measurable excess of sudden death when this antibiotic was chosen rather than an alternative.
What rash means stop immediately?
Any rash that is spreading rapidly, any blistering or peeling, sores or ulcers in the mouth, eyes or genitals, a rash with fever or a general sense of being seriously unwell, or skin that becomes painful and tender to touch. Drugs of this family are among the leading causes of a rare severe blistering reaction in which skin and mucous membranes detach. Flu-like symptoms a day or two before a rash are a recognised warning.
My creatinine went up. Is it damaging my kidneys?
Usually not. Trimethoprim blocks the mechanism by which the kidney secretes creatinine into urine, so the blood level rises while actual filtering function is unchanged, and it reverses within days of finishing. It is a measurement artefact. Genuine kidney injury can also occur, though, so the two need distinguishing — a larger rise, reduced urine output, or a rise in potassium alongside is not the artefact and needs assessment.
Can I take it in pregnancy?
Generally avoided in the first trimester, because it interferes with folate and folate is what allows the structure becoming the brain and spinal cord to close — a process completing around four weeks after conception, before most women know. It is also avoided close to delivery for reasons involving the newborn. Say so before starting, and if a pregnancy is discovered while taking it, seek advice promptly rather than panicking. Alternatives exist.
Will I burn more easily in the sun?
Yes. Skin becomes more sensitive to ultraviolet during the course and for a short time afterwards, so exposure that would normally be harmless can produce a burn. Cover up, use protection, and be particularly careful on holiday, where the combination of a new antibiotic and strong sun catches people out regularly.
I am allergic to sulfa. Does that rule out other drugs?
A previous reaction to this class is a genuine reason to avoid it. It does not automatically rule out every medicine containing sulfur in its structure — the chemistry responsible for the reaction is specific, and cross-reactivity with unrelated drugs is far lower than commonly assumed. What it does mean is telling every prescriber and pharmacist, and having the nature of the original reaction on record.
What should I check before starting?
Your full medicine list, out loud, including things taken for years without incident. Specifically warfarin, methotrexate, anything ending in pril or sartan, potassium-sparing water tablets, sulfonylurea diabetes tablets, phenytoin and digoxin. Also mention pregnancy or the possibility of it, any sulfa allergy, kidney problems and G6PD deficiency. It takes two minutes and it is the whole point of this article.
📑 Sources and editorial
- Pharmacology of sequential folate pathway inhibition by sulfamethoxazole and trimethoprim
- Surveillance data on trimethoprim resistance in urinary isolates and consequent guideline changes
- Literature on co-trimoxazole in Pneumocystis prophylaxis and treatment, and in resistant skin and soft tissue infection
- Evidence on the warfarin interaction and associated bleeding risk
- Reports of severe myelosuppression with concurrent methotrexate
- Population studies on hyperkalaemia and sudden death with co-trimoxazole in patients receiving renin-angiotensin inhibitors
- Interaction data for sulfonylureas, phenytoin and digoxin
- Literature on sulfonamide-associated severe cutaneous adverse reactions and warning features
- Evidence on trimethoprim inhibition of tubular creatinine secretion and pseudo-elevation of serum creatinine
- Guidance on use in pregnancy by trimester, and on G6PD deficiency
- Related reading: urinary tract infections, high potassium, folic acid before conception
- Related products: antibiotics category
- RXshop Editorial Team — reviewed by Emily Chen, MD, Internal Medicine
Medical Disclaimer: The information in this article is for educational and informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek guidance from a qualified healthcare provider with any questions you may have regarding a medical condition, and before starting, stopping or changing any medication.