Sulfamethoxazole is taken as a combination tablet with trimethoprim, typically one double-strength (DS) tablet every 12 hours. The standard DS tablet contains 800 mg of sulfamethoxazole and 160 mg of trimethoprim. It comes in tablet and liquid form, and the way you take it, what you take it with, and how long you stay on it all depend on the infection being treated.
Dosing Schedule and Forms
For most bacterial infections in adults and children weighing 88 pounds (40 kg) or more, the standard dose is one DS tablet every 12 hours. That means roughly one tablet in the morning and one in the evening, spaced about 12 hours apart. If you don’t have the double-strength version, the equivalent is two regular-strength tablets (each containing 400 mg sulfamethoxazole and 80 mg trimethoprim) per dose.
A liquid suspension is also available, dosed at 20 mL (about 4 teaspoons) every 12 hours. This is useful for children or anyone who has trouble swallowing tablets. If you’re using the liquid, shake the bottle well before each dose.
With or Without Food
This medication can be taken on an empty stomach for best absorption, ideally one hour before or two hours after eating. That said, if it causes nausea or an upset stomach, taking it with food is fine. Nausea, vomiting, and diarrhea are among the more common side effects, so many people end up taking it with a meal or snack for comfort.
Drink a full glass of water with each dose. Staying well hydrated throughout your course of treatment helps your kidneys process the drug and reduces the chance of crystal formation in the urinary tract.
How Long to Take It
Treatment length varies by infection. For urinary tract infections and most respiratory infections, a typical course runs 10 to 14 days. Kidney infections (pyelonephritis) are usually treated for 14 days, and prostate infections may require 14 days to several months for chronic cases. Bronchitis is typically treated for 14 days.
Finishing the full prescribed course matters. Every extra day of unnecessary antibiotics raises the risk of side effects and contributes to antibiotic resistance, but stopping early when the infection isn’t fully cleared can allow bacteria to bounce back. Take the medication for exactly as long as it was prescribed, even if you feel better partway through.
What to Do If You Miss a Dose
If you miss a dose, take it as soon as you remember. If it’s almost time for your next scheduled dose, skip the missed one and continue your regular schedule. Don’t double up to make up for a forgotten dose.
Sun Sensitivity
Sulfamethoxazole belongs to a class of drugs called sulfonamides, which are known to cause photosensitivity. This means your skin can burn or develop a rash more easily than usual when exposed to ultraviolet light, sometimes within just a few hours of sun exposure.
While you’re on this medication, use a broad-spectrum sunscreen with at least SPF 30, wear protective clothing and a hat, and try to stay out of direct sun between 10 a.m. and 4 p.m. This applies to tanning beds too, not just natural sunlight. Reflected sunlight off water, sand, or snow can also trigger a reaction.
Important Drug Interactions
If you take a blood thinner like warfarin, sulfamethoxazole-trimethoprim can increase your risk of bleeding significantly. Studies show a two- to threefold increase in the risk of gastrointestinal bleeding compared to other antibiotics in people on warfarin. Your prescriber may need to monitor your blood clotting levels more closely during treatment.
People with diabetes who take sulfonylurea medications (such as glyburide, glipizide, or glimepiride) face a four- to sixfold increased risk of dangerously low blood sugar when adding this antibiotic. The combination boosts insulin release beyond what the diabetes medication alone would cause. If you’re on one of these drugs, watch for symptoms of low blood sugar like shakiness, sweating, and confusion.
Anyone taking methotrexate for arthritis or other conditions should generally avoid this antibiotic. Sulfamethoxazole-trimethoprim reduces the kidney’s ability to clear methotrexate by about 40%, which can lead to a toxic buildup causing serious blood count drops, liver problems, and mouth sores.
Pregnancy and Breastfeeding
For breastfeeding parents, sulfamethoxazole-trimethoprim is generally considered acceptable once the baby is past the newborn period and is healthy and full-term. A large systematic review found no adverse reactions in breastfed infants. However, it should be avoided if the infant is premature, jaundiced, ill, or has a condition called G6PD deficiency, which affects how red blood cells handle certain stresses. If you’re pregnant, discuss the risks and benefits with your prescriber, as sulfonamides carry specific concerns near the end of pregnancy.
Storing the Medication
Keep tablets and liquid at room temperature, between 68°F and 77°F (20°C to 25°C). The liquid suspension should be stored in a light-resistant container, away from direct sunlight, and shaken well before each use. Both forms should be kept out of reach of children.