What Is Sulfamethoxazole/Trimethoprim? Uses, Dosage, and What You Need to Know in 2026

Sulfamethoxazole/trimethoprim is one of the most prescribed antibiotics in the world. It's been in use since 1973, costs just a few dollars as a generic, and treats an unusually wide range of bacterial infections. Here's a comprehensive overview of what it is, how it works, and what you need to know before taking it.

What Is Sulfamethoxazole/Trimethoprim?

Sulfamethoxazole/trimethoprim (TMP-SMX) is a combination antibiotic that contains two drugs that work together: sulfamethoxazole (a sulfonamide antibiotic) and trimethoprim (an antifolate). Brand names include Bactrim, Bactrim DS, Septra, Septra DS, and Sulfatrim. The combination is also known as co-trimoxazole and abbreviated as TMP-SMX, SMX-TMP, TMP-SMZ, or SXT.

It was approved by the FDA in 1973 and remains a cornerstone of antibacterial therapy due to its broad spectrum, affordability, and availability in oral and IV forms. It is not a controlled substance.

What Infections Does TMP-SMX Treat?

FDA-approved indications for TMP-SMX include:

Urinary tract infections (UTIs): One of the most common uses; effective against E. coli, Klebsiella, Enterobacter, Proteus, and other common UTI organisms (check local resistance rates — E. coli resistance is ~15–25% in some areas)

Otitis media (ear infections): Approved for pediatric use only; treats infections caused by S. pneumoniae and H. influenzae

Traveler's diarrhea: For treatment and prophylaxis of diarrhea caused by enterotoxigenic E. coli and Shigella

Shigellosis (bacillary dysentery): Diarrhea caused by Shigella bacteria

Pneumocystis jirovecii pneumonia (PCP): Both treatment and prophylaxis; this is the first-choice drug for PCP in immunocompromised patients (HIV, transplant, cancer)

Acute exacerbation of chronic bronchitis: When caused by susceptible S. pneumoniae or H. influenzae

Common off-label uses include MRSA skin and soft tissue infections, nocardiosis, and prophylaxis against toxoplasmosis in HIV patients.

Available Forms and Dosage Strengths

Single-strength (SS) tablet: 80 mg trimethoprim / 400 mg sulfamethoxazole

Double-strength (DS) tablet: 160 mg trimethoprim / 800 mg sulfamethoxazole — most commonly prescribed for adults

Oral suspension (Sulfatrim): 40 mg TMP / 200 mg SMX per 5 mL — for children and patients who can't swallow tablets

IV injection: For serious infections in hospital settings (severe PCP, hospitalized UTIs)

Typical Dosing by Condition

Uncomplicated UTI (adults): 1 DS tablet (800/160 mg) twice daily for 10–14 days

PCP prophylaxis: 1 DS tablet once daily (or 3 times per week)

Pediatric UTI / otitis media (children ≥2 months): 40 mg/kg/day (SMX component) in 2 divided doses every 12 hours for 10 days

Shigellosis / traveler's diarrhea: 1 DS tablet twice daily for 5 days

Who Should NOT Take TMP-SMX

Known allergy to sulfa drugs or trimethoprim

Infants under 2 months of age

Pregnancy — especially first trimester and near term (due to neural tube defect risk and neonatal bilirubin effects)

Severe kidney or liver disease

Megaloblastic anemia due to folate deficiency

G6PD deficiency (risk of hemolytic anemia)

How to Take TMP-SMX Safely

Take with a full 8 oz glass of water; drink extra water throughout the day.

Can be taken with or without food, but food reduces GI upset.

Complete the full prescribed course, even if you feel better.

Use sunscreen — TMP-SMX increases sun sensitivity.

Once you have your prescription, use medfinder to find which pharmacies near you have it in stock. For a full breakdown of what to watch for, see our TMP-SMX side effects guide.