Sulfamethoxazole/trimethoprim 800/160 mg is a combination antibiotic, commonly known by the brand names Bactrim DS or Septra DS, used to treat a wide range of bacterial infections. The “DS” stands for double strength, and this is the standard adult tablet. It is one of the most commonly prescribed antibiotics in the United States, particularly for urinary tract infections, skin infections, and certain types of pneumonia.
How This Antibiotic Works
This medication combines two drugs that attack bacteria at two different points in the same essential process: making folate. Bacteria need to produce their own folate to build DNA and multiply. Sulfamethoxazole blocks an early step in that production, while trimethoprim blocks a later step. By hitting the same pathway twice, the combination is far more effective than either drug alone. Human cells get folate from food rather than manufacturing it, which is why the drug targets bacteria without doing the same damage to your own cells. The bacterial version of the enzyme trimethoprim targets is roughly four to five times more sensitive to the drug than the human version.
Urinary Tract Infections
UTIs are the single most common reason this medication is prescribed. It works against the bacteria most frequently responsible for bladder infections, including E. coli, which causes the majority of uncomplicated UTIs. For a straightforward bladder infection, a typical course runs three days, though your prescriber may extend that to 7 to 14 days for more complicated infections or those involving the kidneys. It remains a first-line antibiotic for uncomplicated UTIs in many treatment guidelines, though resistance patterns vary by region.
Skin and Soft Tissue Infections
This is one of the go-to oral antibiotics for skin infections caused by MRSA, a type of staph bacteria resistant to many other antibiotics. If you’ve been diagnosed with an abscess, cellulitis, or infected wound and your provider suspects MRSA, there’s a good chance you’ll be prescribed this medication. Dosing for skin infections is weight-based. For someone between about 130 and 200 pounds, the typical regimen is one double-strength tablet three times a day. Heavier patients often need two tablets per dose. Treatment usually lasts 7 to 10 days depending on how severe the infection is and how quickly it responds.
Pneumocystis Pneumonia Prevention and Treatment
People with weakened immune systems, including those with HIV or organ transplants, are vulnerable to a fungal lung infection called Pneumocystis pneumonia (PCP). Sulfamethoxazole/trimethoprim is the preferred drug both for treating active PCP and for preventing it in the first place. For prevention, the standard dose is one double-strength tablet once daily. This prophylactic use is one of the most important applications of the drug in modern medicine, as PCP can be life-threatening in immunocompromised patients.
Other Approved Uses
Beyond UTIs and PCP, this antibiotic carries several other FDA-approved uses:
- Traveler’s diarrhea caused by toxin-producing E. coli, a common cause of illness in people visiting developing countries.
- Shigellosis, a bacterial infection of the intestines that causes severe diarrhea, fever, and stomach cramps.
- Flare-ups of chronic bronchitis in adults, when the infection is caused by susceptible bacteria.
- Middle ear infections in children, though this use is more common with the liquid formulation than the 800/160 tablet.
Common Side Effects
Most people tolerate this antibiotic well, but side effects do occur. Nausea, vomiting, diarrhea, and loss of appetite are the most frequent complaints. Some people develop a mild skin rash. Because the drug increases sun sensitivity, you may sunburn more easily while taking it. Drinking plenty of water throughout the course helps protect your kidneys, since the drug can crystallize in concentrated urine.
Serious Reactions to Watch For
Severe reactions are uncommon but important to recognize. The most concerning is a group of serious skin reactions, including Stevens-Johnson syndrome, which can develop weeks to months after starting the medication. Warning signs include a spreading rash with blistering, peeling skin, red or purple skin lesions (sometimes with a dark center), or flu-like symptoms accompanying any rash. Swelling of the face, lips, or lymph nodes alongside a rash also warrants immediate medical attention.
The drug can also affect blood cell production. Because it interferes with folate metabolism, it can lower red blood cell, white blood cell, or platelet counts over time. Unusual fatigue, dizziness, increased bruising, or shortness of breath during treatment are signs worth reporting promptly.
Potassium Levels and Kidney Function
Trimethoprim has an underappreciated effect: it reduces the kidneys’ ability to excrete potassium, cutting urinary potassium loss by about 40%. For most healthy adults, this doesn’t cause problems. But for people already taking medications that raise potassium levels, the combination can become dangerous. A study published in The BMJ found that patients taking the potassium-sparing diuretic spironolactone who were prescribed this antibiotic had roughly 12 times the risk of being hospitalized for dangerously high potassium compared to those prescribed a different antibiotic. The double-strength 800/160 tablet carried a slightly higher risk than the single-strength version.
Kidney function also determines whether you can take the full dose. If your kidneys filter at a reduced rate, the drug accumulates. At moderate kidney impairment (filtration rate between 15 and 30 mL/min), the dose is typically cut in half. Below 15 mL/min, the drug is generally not recommended at all.
Who Should Avoid This Medication
This antibiotic is not appropriate for everyone. People with a sulfa allergy should not take it, as sulfamethoxazole belongs to the sulfonamide drug class. It’s also avoided in late pregnancy because of potential effects on the newborn, and in infants under two months old. Anyone with severe liver disease or a history of low blood counts triggered by this medication should use an alternative. If you have a folate deficiency or are taking other medications that affect folate, the risk of blood-related side effects increases.