Sulfamethoxazole/Trimethoprim

About Sulfamethoxazole/Trimethoprim

Sulfamethoxazole is a sulfonamide, and with trimethoprim it stops the growth of susceptible bacteria; like other antibiotics, the combination does not treat viral infections such as the common cold. The label covers urinary tract infections (for a first uncomplicated episode it recommends a single effective antibiotic instead), acute otitis media in children and acute exacerbations of chronic bronchitis in adults when the prescriber judges it offers an advantage, shigellosis, travelers’ diarrhea in adults caused by enterotoxigenic E. coli, and treatment of Pneumocystis jirovecii pneumonia and its prevention in people with weakened immune systems at increased risk. Adults usually take one DS tablet or two regular tablets every 12 hours: 10 to 14 days for urinary infections, 5 days for shigellosis or travelers’ diarrhea and 14 days for bronchitis flares; the adult preventive dose for Pneumocystis pneumonia is one DS tablet a day. Children 2 months and older are dosed by weight. Skin infections, including MRSA, are not an FDA-approved use, and the prescriber decides.

There is no boxed warning. Serious and sometimes fatal reactions include Stevens-Johnson syndrome, toxic epidermal necrolysis, DRESS and other severe skin reactions, fulminant liver necrosis, agranulocytosis, aplastic anemia and other blood disorders, lung injury, anaphylaxis and circulatory shock. The label says to stop it at the first skin rash or sign of a serious reaction; sore throat, fever, joint pain, cough, chest pain, shortness of breath, pallor, purple spots or jaundice may be early signs. It must not be taken with a sulfonamide or trimethoprim allergy, past drug-induced immune thrombocytopenia from either, megaloblastic anemia from folate deficiency, marked liver damage, severe kidney insufficiency when kidney function cannot be monitored, or dofetilide, or by infants under 2 months. High doses (as for Pneumocystis pneumonia), and even usual doses with kidney insufficiency, potassium disorders or drugs that raise potassium, can cause high potassium, which is closely monitored; low sodium and low blood sugar can also occur. It should not be used for group A streptococcal infections. Nausea, vomiting, loss of appetite, rash and hives are the most common side effects.

The February 2025 Bactrim label says to avoid it with impaired kidney or liver function, possible folate deficiency (for example in older adults, with chronic alcoholism, anticonvulsant treatment, malabsorption or malnutrition), severe allergies or bronchial asthma, porphyria or thyroid dysfunction; hemolysis can occur with G6PD deficiency. Some epidemiologic studies suggest that use in pregnancy may be linked to birth defects, particularly neural tube, heart and urinary tract defects, oral clefts and club foot, and anyone who takes it while pregnant is told of the potential hazards to the fetus; caution is advised while breastfeeding because of the risk of kernicterus. Drinking enough fluid helps prevent crystals and stones in the urine, and MedlinePlus notes it may make skin sensitive to sunlight. Septra, Septra DS and the Bactrim and Septra suspensions are discontinued.

Taking Sulfamethoxazole/Trimethoprim

How to take it

Usually every 12 hours, as prescribed, with plenty of fluids to help prevent crystals and kidney stones. Shake the suspension well before each dose. Finish the course even if you feel better, but stop and call the prescriber at the first sign of a rash, and protect your skin from the sun.

If you miss a dose

Take it as soon as you remember, unless it is almost time for the next dose; then skip the missed dose and carry on as usual. Do not take a double dose.

Storage

At room temperature in a tightly closed, light-resistant container, away from excess heat and moisture, out of reach of children.

Interactions to know about

Dofetilide must not be taken with it. The label advises avoiding methotrexate, cyclosporine, diuretics, ACE inhibitors, amantadine, indomethacin and pyrimethamine; warfarin, phenytoin, digoxin, diabetes medicines, tricyclic antidepressants, procainamide and zidovudine need monitoring; and leucovorin is avoided during Pneumocystis pneumonia treatment.

This summarizes the official prescribing information. Your prescriber’s and pharmacist’s instructions, and the leaflet that comes with your medicine, take priority.

Why the cash price for Sulfamethoxazole/Trimethoprim differs from pharmacy to pharmacy

Generic regular and DS tablets come from several makers, so quotes for the same course differ by each pharmacy’s dispensing fee and markup and by whether the combination is on its flat-price generic list. A dose written as one DS tablet or as two regular tablets is quoted as a different quantity, and a prescription that names Bactrim and blocks substitution is filled with the brand.

The suspension, generic or Sulfatrim Pediatric, is quoted by bottle, and a child’s weight-based dose sets how much the course needs.

Ways to pay less for Sulfamethoxazole/Trimethoprim without insurance

• Ask whether sulfamethoxazole-trimethoprim is on the pharmacy’s flat-price generic list, and compare more than one pharmacy.
• If the prescription names Bactrim or Bactrim DS, check that it allows a generic.
• For long-term Pneumocystis prevention, ask the prescriber whether a ninety-day supply is appropriate.
• For a child’s suspension, ask how many bottles the full course needs, so the quote covers all of it.
• Upload the prescription to Script Unlock so licensed pharmacies can quote their cash price. You pay the pharmacy directly.

Questions to ask your pharmacist about Sulfamethoxazole/Trimethoprim

1. 1.Is it safe with my sulfa allergy or a past rash from an antibiotic?
2. 2.Do my kidneys or my blood pressure medicine raise my risk of high potassium?
3. 3.Does it interact with my warfarin, diabetes medicine or methotrexate?
4. 4.Which rash, sore throat, fever or bruising means I should stop it and call?

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