Bactrim and Septra (TMP-SMX).
Sulfamethoxazole/trimethoprim (TMP-SMX) is a fixed combination of two folate-pathway inhibitors. Sold under several brand names — most commonly Bactrim and Septra in North America and Co-trimoxazole elsewhere — it is the most prescribed sulfa drug today and the drug behind most "sulfa allergy" labels.
- Components
Sulfamethoxazole(a sulfa antibiotic) +trimethoprim(a dihydrofolate reductase inhibitor).- Brand names
Bactrim,Septra,Co-trimoxazole(UK/Europe), and others.- Common indications
- UTI, MRSA skin/soft tissue, PCP prophylaxis and treatment, toxoplasmosis, Stenotrophomonas.
- Mechanism
- Sequential blockade of bacterial folate synthesis — bactericidal in many organisms.
What it is
TMP-SMX combines sulfamethoxazole — a sulfa antibiotic that inhibits dihydropteroate synthase — with trimethoprim, which inhibits the next enzyme in the bacterial folate pathway, dihydrofolate reductase. Each component alone is bacteriostatic. The combination is bactericidal in many organisms because the folate pathway is blocked at two consecutive steps, and the resulting folate deficit is more severe than either drug alone produces. How sulfa drugs work covers the mechanism.
What it is used for
Uncomplicated urinary tract infection remains a common indication in non-pregnant adults where local resistance permits. National guidelines often list it among first-line options for uncomplicated cystitis.
MRSA skin and soft-tissue infection. TMP-SMX is widely used for community-acquired methicillin-resistant Staphylococcus aureus infections in skin and soft tissue, often as a first-line oral option in many settings.
Pneumocystis jirovecii pneumonia (PCP / PJP). TMP-SMX is the first-line agent for both prophylaxis and treatment in immunocompromised patients — including those with HIV, organ transplant recipients, and patients on certain immunosuppressive drugs. Sulfa and HIV covers the HIV context in detail.
Toxoplasmosis. TMP-SMX is sometimes used as an alternative to pyrimethamine + sulfadiazine for toxoplasmosis treatment and prophylaxis.
Other. Stenotrophomonas maltophilia infections (TMP-SMX is the first-line agent), some Nocardia infections, certain travelers' diarrhoea (less than historically, due to resistance), and others.
Common adverse effects
The most common adverse effects are gastrointestinal (nausea, anorexia, abdominal discomfort) and a delayed maculopapular rash, often appearing one to two weeks into a course. Photosensitivity is recognised. Drug fever can occur. Less common but important effects include:
Hyperkalemia — TMP-SMX has a structural similarity to amiloride and blocks epithelial sodium channels in the distal nephron, raising potassium. The effect is dose-related and more pronounced in patients with kidney impairment or on other potassium-sparing drugs.
An elevated serum creatinine often occurs on TMP-SMX without a true fall in glomerular filtration rate. Trimethoprim competitively inhibits creatinine secretion in the proximal tubule. The creatinine rises modestly, but the underlying GFR is unchanged. Kidney effects covers this and the related real renal concerns.
Hyponatremia — particularly in older patients and on higher doses.
Hematologic effects — leukopenia, thrombocytopenia, and rarely agranulocytosis or megaloblastic anemia. The anemia is folate-related and is one reason folinic acid (leucovorin) is sometimes co-prescribed in high-dose long-course use.
Hepatic injury — uncommon but reported, ranging from mild transaminase elevation to severe hepatocellular damage.
Severe reactions
The serious adverse reactions include Stevens-Johnson syndrome and toxic epidermal necrolysis, DRESS, drug-induced liver injury, and rare interstitial nephritis. Anaphylaxis to TMP-SMX exists but is uncommon. Mild vs severe reactions covers the distinguishing features.
The HIV context
People with HIV — particularly with low CD4 counts — have markedly higher rates of cutaneous reactions to TMP-SMX than the general population. Despite this, TMP-SMX is the first-line agent for PCP/PJP prophylaxis and treatment because alternatives are less effective. Desensitisation protocols exist for selected patients with reaction history; they are specialist procedures and this site does not describe them. Sulfa and HIV covers the topic.
Pregnancy
TMP-SMX in pregnancy is shaped by two specific concerns. Trimethoprim is a folate antagonist, and first-trimester exposure has been associated with neural tube and other defects in some studies. Sulfamethoxazole near term carries a theoretical risk of kernicterus (bilirubin displacement) in the newborn, particularly in late pregnancy and in preterm infants. Decisions are case-by-case; sulfa and pregnancy covers the points.
Photosensitivity
TMP-SMX is associated with photosensitive reactions. Routine sun precautions — protective clothing, broad-spectrum sunscreen, avoiding peak UV — reduce the risk. Photosensitivity covers it.
Interactions
TMP-SMX has clinically important drug interactions. Of particular note: it potentiates warfarin (raising INR), it raises the levels of several oral hypoglycemics including sulfonylureas (risk of hypoglycemia), it interacts with methotrexate (risk of myelosuppression — generally avoid co-prescription except under specialist supervision), and it can raise potassium when combined with ACE inhibitors, ARBs, or potassium-sparing diuretics. Pharmacist review at prescribing time matters.