Sulfamethoxazole/trimethoprim 800/160 mg, commonly called TMP-SMX or by the brand name Bactrim DS, is a combination antibiotic prescribed for a wide range of bacterial infections. Urinary tract infections are the most common reason it lands on a prescription pad, but doctors also reach for it to treat skin and soft tissue infections, certain respiratory infections, traveler’s diarrhea, and specific gut pathogens like Shigella. In people with weakened immune systems, it plays a critical role as a preventive medication against a dangerous fungal-like lung infection called Pneumocystis pneumonia. What makes this drug unusual among antibiotics is how the two ingredients work together, and why that matters for when it works and when it doesn’t.
How the Two Ingredients Work Together
The “800/160” in the name refers to 800 mg of sulfamethoxazole and 160 mg of trimethoprim in each tablet. These two drugs attack bacteria at two consecutive steps in the same biochemical pathway. Sulfamethoxazole blocks production of a precursor to tetrahydrofolate (a form of folic acid bacteria need to build DNA), while trimethoprim blocks the very next step, preventing conversion of dihydrofolate into the usable form.1PubMed Central. Mutual potentiation drives synergy between trimethoprim and sulfamethoxazole Human cells get their folate from food rather than making it from scratch, so this double blockade hits bacteria hard without directly poisoning your own cells. The result is a synergistic effect: the combination is often bactericidal (kills bacteria outright) against organisms that either drug alone would merely slow down.
This dual mechanism gives TMP-SMX a broad reach. It works against many common gram-positive bacteria (including certain staph and strep species) and gram-negative bacteria (including many of the bugs behind urinary and intestinal infections). That breadth is a big part of why physicians have relied on it since the 1970s.
Urinary Tract Infections
UTIs remain the bread-and-butter indication for TMP-SMX. For uncomplicated bladder infections in women, a short course clears the infection in the vast majority of cases. In one early trial, single-dose TMP-SMX eradicated the infection in about 93% of women with confirmed bacteriuria.2PubMed. Single-dose therapy with trimethoprim-sulfamethoxazole for urinary tract infection in women A later randomized trial comparing TMP-SMX against fluoroquinolone antibiotics found clinical success rates around 95% for a three-day course.3PubMed. A randomized trial of short-course ciprofloxacin, ofloxacin, or trimethoprim/sulfamethoxazole for the treatment of acute urinary tract infection in women Those numbers hold up across multiple studies, and the short treatment course, low cost, and limited side-effect profile keep TMP-SMX near the top of guideline recommendations for uncomplicated UTIs where local resistance rates remain below about 20%.
That resistance caveat matters. In some regions, a significant share of the E. coli strains that cause UTIs have developed resistance to TMP-SMX, which is why your doctor may choose a different antibiotic if local resistance patterns are unfavorable or if you’ve recently taken TMP-SMX. When the bacteria are susceptible, though, few antibiotics match TMP-SMX for convenience and efficacy in uncomplicated bladder infections.
Skin and Soft Tissue Infections
TMP-SMX has become one of the go-to oral antibiotics for skin infections caused by methicillin-resistant Staphylococcus aureus (MRSA). When community-acquired MRSA exploded in prevalence in the 2000s, clinicians needed an oral option that worked against these resistant staph bacteria, and TMP-SMX fit the bill. A systematic review of observational and randomized studies found strong support for using TMP-SMX to treat impetigo, purulent cellulitis, abscesses, and wound infections, including those involving staph.4PubMed Central. Sulfamethoxazole-Trimethoprim (Cotrimoxazole) for Skin and Soft Tissue Infections Including Impetigo, Cellulitis, and Abscess
In a randomized trial comparing TMP-SMX head-to-head with clindamycin (another common choice for MRSA skin infections), cure rates at follow-up were essentially identical at around 92% for both drugs.5PubMed Central. A Randomized Trial of Clindamycin Versus Trimethoprim-sulfamethoxazole for Uncomplicated Wound Infection One question that came up was whether doubling the dose would improve outcomes for MRSA skin infections. A study that compared standard dosing (one DS tablet twice daily) against double the standard dose found no meaningful difference in cure rates, with roughly three-quarters of patients in both groups achieving clinical resolution.6PubMed Central. Dose of trimethoprim-sulfamethoxazole to treat skin and skin structure infections caused by methicillin-resistant Staphylococcus aureus So for most skin infections, the standard 800/160 mg dose taken twice daily appears to be sufficient.
Traveler’s Diarrhea and Intestinal Infections
TMP-SMX has a long track record for treating acute diarrheal illness, particularly in travelers. A randomized, placebo-controlled trial of U.S. adults who developed acute diarrhea in Mexico found that even a single high dose of TMP-SMX shortened the average duration of illness from about 59 hours to 28 hours. When TMP-SMX was combined with loperamide (an over-the-counter anti-motility drug), the average duration dropped to just one hour.7JAMA. Treatment of Traveler’s Diarrhea With Sulfamethoxazole and Trimethoprim and Loperamide That said, bacterial resistance patterns in many popular travel destinations have shifted over the decades, and fluoroquinolones or azithromycin are now more commonly recommended for empiric treatment. TMP-SMX still works well when the offending bacteria are susceptible.
TMP-SMX also has a specific role in shigellosis, the dysentery-like illness caused by Shigella bacteria. In areas where Shigella strains carry resistance to ampicillin and other first-line drugs, TMP-SMX has historically been considered one of the best available treatments. Clinical trials in hospitalized children showed that TMP-SMX cleared stool cultures in under two days and resolved diarrhea in about three days, performing comparably to ampicillin when the bacteria were susceptible to both drugs.8PubMed. Trimethoprim-sulfamethoxazole therapy for shigellosis9Pediatrics. Trimethoprim-Sulfamethoxazole Compared to Ampicillin in the Treatment of Shigellosis
Respiratory Infections
For flare-ups of chronic bronchitis (often called acute exacerbations of COPD), TMP-SMX is sometimes prescribed, especially where common respiratory pathogens remain susceptible. In a trial of COPD patients sick enough to need mechanical ventilation, TMP-SMX was found to be not inferior to ciprofloxacin.10PubMed. Standard versus newer antibacterial agents in the treatment of severe acute exacerbation of chronic obstructive pulmonary disease: a randomized trial of trimethoprim-sulfamethoxazole versus ciprofloxacin That result surprised some clinicians who assumed newer antibiotics would outperform it. In outpatient settings, one comparative trial showed a lower clinical success rate for TMP-SMX (about 52%) against a fluoroquinolone for exacerbations of chronic bronchitis, though differences in study design and patient severity make direct comparison tricky.11PubMed. Ofloxacin versus trimethoprim-sulfamethoxazole in the treatment of patients with acute exacerbation of chronic bronchitis
TMP-SMX is not a first-line choice for typical community-acquired pneumonia or strep throat. Its respiratory niche is narrower: bronchitis exacerbations, certain types of sinusitis, and, most critically, treatment and prevention of Pneumocystis pneumonia.
Preventing Pneumocystis Pneumonia in Immunocompromised Patients
One of the most important uses of TMP-SMX has nothing to do with common bacterial infections. In people whose immune systems are suppressed, whether from HIV/AIDS, organ transplantation, cancer chemotherapy, or long-term corticosteroid use, a fungal-like organism called Pneumocystis jirovecii can cause a severe and potentially fatal lung infection known as Pneumocystis pneumonia (PCP). TMP-SMX is the standard prophylactic medication to prevent this.12PubMed Central. Pneumocystis Pneumonia in Locally Advanced Breast Cancer Despite Prophylactic Use of Trimethoprim-Sulfamethoxazole During Prednisolone Treatment for a Pembrolizumab-Induced Immune-Related Adverse Event: A Case Report
For prophylaxis, the typical regimen is one DS tablet daily or three times per week, rather than twice daily as used for active infections. This preventive use has been one of the major life-saving interventions in HIV medicine. TMP-SMX prophylaxis also provides a side benefit: protection against several other opportunistic infections including toxoplasmosis and nocardiosis. Patients who switch to alternative prophylactic agents because they cannot tolerate TMP-SMX lose some of that broader protection.13Emerging Infectious Diseases. Nocardiosis in Immunocompromised Patients on Alternative Pneumocystis Prophylaxis
Side Effects Worth Knowing About
Most people tolerate TMP-SMX without trouble, but the drug does carry a handful of side effects that range from merely annoying to genuinely dangerous. The common ones are mild: nausea, loss of appetite, rash, and sometimes diarrhea. A few deserve closer attention.
Trimethoprim can raise your blood potassium level. It does this by blocking sodium channels in the kidney in a way that resembles the action of potassium-sparing diuretics. In healthy people with normal kidneys, this rarely causes problems. But in older adults, people with kidney disease, or anyone already taking medications that raise potassium (like ACE inhibitors or spironolactone), the combination can push potassium into a dangerous range.14PubMed. Renal mechanism of trimethoprim-induced hyperkalemia If you’re in one of those groups, your doctor may check your potassium level during treatment.
Serious skin reactions, including Stevens-Johnson syndrome (SJS), are rare but can occur. People with HIV are at higher risk of adverse reactions to TMP-SMX compared to the general population, and SJS is one of the more feared complications.15PubMed Central. Trimethoprim-sulfamethoxazole-induced Steven Johnson syndrome in an HIV-infected patient The sulfamethoxazole component is the usual culprit, and individuals who metabolize sulfonamides more slowly (so-called slow acetylators) appear to face a higher risk of these idiosyncratic skin reactions.16Journal of the American Pharmacists Association. Sulfamethoxazole-trimethoprim-induced Stevens-Johnson syndrome in a NAT2 slow acetylator: A case for pre-emptive pharmacogenomic screening? Any new widespread rash, blistering, or mouth sores that develop while taking TMP-SMX warrant immediate medical attention.
Drug Interactions That Can Be Dangerous
Two drug interactions stand out for TMP-SMX because the consequences can be severe.
The first is warfarin. TMP-SMX can amplify warfarin’s blood-thinning effect through multiple mechanisms, including displacement of warfarin from its binding sites on blood proteins and inhibition of the enzymes that metabolize warfarin.17PubMed Central. Interaction between warfarin and sulphamethoxazole The clinical impact is real: multiple studies of older adults on long-term warfarin therapy have found that adding TMP-SMX roughly doubles to triples the risk of a bleeding event requiring hospitalization, and about a third of patients prescribed the combination see their INR (a measure of blood-clotting time) climb to dangerously high levels.18PubMed Central. Warfarin and Antibiotics: Drug Interactions and Clinical Considerations If you’re on warfarin and get prescribed TMP-SMX, expect your doctor to monitor your INR more closely and possibly reduce your warfarin dose during the antibiotic course.
The second is methotrexate, a drug used for autoimmune diseases and certain cancers. Both methotrexate and TMP-SMX interfere with folate metabolism, so combining them creates a kind of folate double-whammy that can suppress bone marrow production of blood cells. TMP-SMX also impairs the kidney’s ability to clear methotrexate from the body. Even at the low methotrexate doses used for rheumatoid arthritis, this combination has caused life-threatening drops in blood cell counts.19American Journal of Respiratory and Critical Care Medicine. C53-13 Methotrexate and Trimethoprim-Sulfamethoxazole Interaction Causing Severe Pancytopenia If you take methotrexate for any reason, make sure every prescriber and pharmacist knows before TMP-SMX is dispensed.
Staying Hydrated and Protecting Your Kidneys
An underappreciated practical concern with TMP-SMX is kidney crystal formation. The sulfamethoxazole component and its metabolites can crystallize in the urine, particularly when urine is concentrated and acidic. A case series investigating sulfamethoxazole-induced crystal nephropathy found that key risk factors included concentrated urine (low fluid intake), low blood albumin, pre-existing kidney disease, and high doses of the drug.20PubMed Central. Sulfamethoxazole-induced crystal nephropathy: characterization and prognosis in a case series The practical takeaway is straightforward: drink plenty of water while taking TMP-SMX. Adequate hydration dilutes the urine and reduces the chance of crystals forming in the kidney tubules. This is especially true for anyone taking higher-than-standard doses, such as the doses used to treat PCP rather than prevent it.
TMP-SMX can also nudge your creatinine level upward on blood tests without actually damaging the kidneys. Trimethoprim interferes with the kidney’s secretion of creatinine, making it look like kidney function has worsened when it hasn’t. If your doctor notices a small creatinine rise during treatment, that artifact is usually the explanation, not true kidney injury.
Why Resistance Is a Growing Concern
Bacteria have developed several strategies for dodging TMP-SMX. Some produce altered versions of the target enzymes that the drugs cannot bind to, and these resistance genes can sit on mobile genetic elements (plasmids) that spread easily between bacterial species.21PubMed. Mechanisms of resistance to trimethoprim, the sulfonamides, and trimethoprim-sulfamethoxazole E. coli resistance to TMP-SMX varies geographically, but in parts of the United States it has crept above 20% in some communities, which is generally considered the threshold at which a drug should no longer be used empirically for UTIs. Your doctor should have access to local antibiogram data, a running tally of what bugs in your area are resistant to what drugs, to make the right call.
Despite these resistance trends, TMP-SMX remains effective against many pathogens and is still widely used worldwide. Its low cost, oral availability, and generally mild side-effect profile give it staying power in an era when many newer antibiotics are far more expensive and sometimes no more effective for common infections.
The Drug’s Effect on Gut Bacteria
All antibiotics disrupt the gut microbiome to some degree, but TMP-SMX appears to be gentler than many alternatives in one specific respect: the risk of Clostridioides difficile infection (C. diff). An analysis of the FDA’s adverse-event database found that the trimethoprim-sulfonamide class had the lowest reported association with C. diff among all antibiotic classes studied, far lower than clindamycin, which had the highest.22PubMed Central. Clostridium difficile Infection Risk with Important Antibiotic Classes: An Analysis of the FDA Adverse Event Reporting System C. diff can cause debilitating and sometimes dangerous diarrhea, so this relatively lower risk is a meaningful practical advantage, especially for older adults or hospitalized patients who are at elevated risk.
Brain Penetration and Unusual Infections
One pharmacological property that sets TMP-SMX apart from many other oral antibiotics is its ability to cross into the cerebrospinal fluid (the fluid surrounding the brain and spinal cord). In adults with intact meninges, trimethoprim reaches cerebrospinal fluid at roughly 18% of blood levels, and sulfamethoxazole at about 12%.23PubMed Central. Pharmacokinetics of trimethoprim and sulfamethoxazole in serum and cerebrospinal fluid of adult patients with normal meninges Those numbers improve when the meninges are inflamed, as occurs during meningitis. This makes TMP-SMX one of the few oral antibiotics that can be used for central nervous system infections caused by susceptible organisms, such as Nocardia or Listeria, which is why infectious-disease specialists sometimes reach for it in clinical scenarios where most oral drugs would fall short.
This same penetration is part of why TMP-SMX is the preferred drug for treating brain abscesses caused by Nocardia, and for toxoplasmosis in HIV patients when the standard treatment (pyrimethamine plus sulfadiazine) is unavailable. In these rare but serious infections, few alternatives match TMP-SMX’s combination of oral bioavailability, central nervous system access, and anti-infective spectrum.
Who Should Avoid It
TMP-SMX is contraindicated in people with a documented allergy to sulfonamide antibiotics, a group of drugs that includes sulfamethoxazole but also older medications like sulfasalazine. In newborns, sulfonamides carry a theoretical concern about displacing bilirubin from its binding sites on albumin, which could raise the risk of a brain-damaging condition called kernicterus. A review of the clinical evidence found no reported cases of kernicterus with standard TMP-SMX doses in neonates, and concluded that short oral courses are unlikely to cause the problem, but the drug is still generally avoided in the first two months of life as a precaution.24PubMed Central. Cotrimoxazole and neonatal kernicterus: a review Pregnant women in their third trimester are also typically steered away from TMP-SMX due to related bilirubin concerns for the newborn.
People with severe kidney impairment need dose adjustments, and those with significant liver disease or certain blood disorders (like megaloblastic anemia from folate deficiency) should generally not take TMP-SMX. If you have glucose-6-phosphate dehydrogenase (G6PD) deficiency, the sulfamethoxazole component can trigger hemolytic anemia, a breakdown of red blood cells. Your doctor should check for this if there’s reason to suspect it.