Cotrimoxazole: what it does and what to watch

Also known as: Trimethoprim-sulfamethoxazole, Co-trimoxazole · Combination sulfonamide antibacterial (trimethoprim plus sulfamethoxazole) · Updated 2026-08-19 · Educational — not medical advice

Illustrative — bacteria with whip-like tails, shown schematically

In plain language: This medicine combines two drugs that each block a different step in the same pathway bacteria use to build folate, a nutrient they need to make genetic material and multiply. Sulfamethoxazole blocks an earlier step in that pathway, while trimethoprim blocks a later one, so together they shut folate production down far more completely than either could alone. Human cells get folate from food rather than manufacturing it this way, which is part of why the combination can target bacteria selectively.

What it's used for

  • Urinary tract infections caused by susceptible bacteria
  • Certain respiratory infections
  • Prevention of specific opportunistic infections in people with weakened immune systems
  • Some traveler's diarrhea and gut infections

Common side effects

  • Nausea and vomiting
  • Loss of appetite
  • Headache
  • Mild skin rash
  • Diarrhea

Get medical help if…

A spreading rash with blistering, peeling skin, or mouth sores, which can signal a rare but severe skin reaction and needs urgent attention

Signs of a serious allergic reaction such as facial or throat swelling and difficulty breathing

Unusual bruising, bleeding, sore throat, or fever suggesting a drop in blood cell counts

Yellowing of the skin or eyes indicating possible liver involvement

Marked reduction in urination or swelling, which can point to kidney effects

Lab values it can move

Few guides tell you this part: medicines show up in blood work. If you take Cotrimoxazole and a value below looks off on your report, this may be part of the story — never stop a medicine without your doctor.

This medicine can raise blood potassium, an effect that becomes more noticeable at higher doses or in people with reduced kidney function.

Potassium

Creatinine readings can rise somewhat, partly because the medicine blocks a kidney transport step rather than truly reducing filtering, though true kidney effects can also occur.

Creatinine

Because both components interfere with folate-related pathways, folate levels can be affected, which matters most for people who are already folate-deficient or pregnant.

Folate (Vitamin B9)

A reduction in platelet count is an uncommon but recognized effect, usually reversible after stopping the medicine.

Platelet Count

Good to know

  • People with a known sulfa allergy should not take this medicine, and any past severe skin reaction to a sulfa drug should be flagged before starting
  • Report a spreading rash, blistering, or mouth sores immediately, since it may be an early sign of a severe skin reaction
  • This medicine can raise blood levels of methotrexate and certain other drugs, so all current medicines should be reviewed with a prescriber first
  • Extra caution and monitoring apply for people with kidney disease or on medicines that raise potassium
  • Drink adequate fluids during treatment
  • Generally avoided in pregnancy — especially the first trimester (it works against folate) and near delivery (newborn jaundice risk) — and used cautiously while breastfeeding a young or premature baby; alternatives usually exist.

Questions people ask

What does a sulfa allergy have to do with this medicine?

One of its two components, sulfamethoxazole, belongs to the sulfonamide (sulfa) drug family. People with a known sulfa allergy can react to it, sometimes with a mild rash but occasionally with a severe, blistering skin reaction that needs emergency care. Anyone with a history of sulfa allergy should tell their prescriber before this medicine is considered.

Why does it matter if someone also takes methotrexate?

Both drugs affect folate-related pathways, and this medicine can also reduce how quickly the body clears methotrexate, allowing it to build up to higher, more toxic levels. Combining them can increase the risk of blood cell suppression and other serious effects, so a prescriber needs to know about all current medicines beforehand.

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