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Sulfamethoxazole and trimethoprim

Brand names: Bactrim, Septra, SMZ-TMP

Sulfamethoxazole-trimethoprim (Bactrim) is a sulfa antibiotic for urinary tract infections, some skin infections, and other infections.

Brand namesBactrim, Septra, SMZ-TMP
Type of medicineSulfa antibiotic
Prescription?Prescription
ImportantThis page is general information, not a prescription. Don't start, stop, or change a medicine without talking to your provider or pharmacist.

Overview

It combines two antibiotics that work together. It doesn't work on viruses.

What it's used for

  • Urinary tract infections
  • Skin infections, including some MRSA
  • Traveler's diarrhea
  • A kind of pneumonia in people with weak immune systems

How to take it

  • Take it with a full glass of water and drink plenty of fluids.
  • Finish the full course even if you feel better.

Your dose depends on your health and other medicines. Always follow your prescription label, and ask your pharmacist if anything is unclear.

Side effects

Common side effects

  • Nausea
  • Less appetite
  • Rash
  • Sunburn more easily
Call 911A blistering, peeling, or widespread red rash, especially with fever
Call 911Fever, sore throat, and rash together
Call your provider todayUnusual bruising, bleeding, or pale skin
Call your provider todayMuscle weakness or a slow or irregular heartbeat (high potassium)
Call your provider todayYellow skin or eyes, or peeing much less
Call your provider todaySevere watery or bloody diarrhea, even weeks after finishing (possible C. diff infection)

Warnings

  • Don't take it if you're allergic to sulfa drugs.
  • Not for babies under 2 months.
  • Tell your provider about kidney or liver disease, or folate deficiency.

Other medicines, foods, and alcohol

  • It strongly raises the effect of warfarin; your INR needs checking.
  • ACE inhibitors, ARBs, and spironolactone add to the risk of high potassium.
  • It can raise methotrexate, phenytoin, and some diabetes pill effects (low blood sugar).

Tell your provider and pharmacist about every medicine, vitamin, and supplement you take.

Pregnancy and breastfeeding

Usually avoided in pregnancy, especially early and near delivery. Talk with your provider.

Missed doses, overdose, and storage

If you miss a dose

Take it as soon as you remember. If it's almost time for the next dose, skip the missed one. Don't double up, and still finish the full course.

If someone takes too much

Call Poison Control at 1-800-222-1222 or get help online at poison.org. Call 911 if the person collapses, has a seizure, has trouble breathing, or can't be woken.

Storing it

Store at room temperature, away from heat and moisture (not in the bathroom), and out of reach of children and pets.

Conditions it's used for

Sources

Last updated 2026-11-13.