Trimethoprim

in BACTRIM

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What it isDihydrofolate reductase inhibitor, usually paired with sulfamethoxazole; urinary infection.

Why this oneThe sequential folate blockade with sulfamethoxazole is synergistic.

What limits itRaises creatinine by blocking tubular secretion without changing GFR, and causes hyperkalemia by blocking the epithelial sodium channel.

FDA-approved for

UTIs (~1980)

FDA label

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FDA dosing	Population	Start	Target	Max
Uncomplicated UTI, initial episodes (susceptible organisms)	Adults	100 mg BID or 200 mg QD x 10 days	100 mg BID or 200 mg QD x 10 days	200 mg/day

Renal CrCl 15-30: 50 mg q12h; CrCl <15: not recommended

Hepatic No dose guidance; give with caution in hepatic impairment

Instructions

Complete the full 10-day course even if feeling better

Contraindications

Documented megaloblastic anemia due to folate deficiency

Cautions

Hematologic toxicity: may interfere with hematopoiesis (esp. large doses/prolonged use); sore throat, fever, pallor, or purpura may herald serious blood disorder — obtain CBC and discontinue if a formed element significantly reduced
Give with caution in possible folate deficiency (folates may be co-administered without loss of antibacterial effect)
Rare serious hypersensitivity reactions (exfoliative dermatitis, SJS, TEN, anaphylaxis)
C. difficile-associated diarrhea (mild to fatal colitis), up to >2 months post-treatment
Caution in renal or hepatic impairment
Elderly: greater renal-impairment risk; consider monitoring potassium and creatinine clearance (hyperkalemia case reports)
Effectiveness as single agent not established in children <12 y; safety not established <2 months

Adverse reactions

Rash and pruritus (most common; 2.9-6.7% at recommended doses), phototoxic skin eruptions
GI: epigastric distress, nausea, vomiting, glossitis; rare cholestatic jaundice; transaminase/bilirubin elevation
Hematologic: thrombocytopenia, leukopenia, neutropenia, megaloblastic anemia, methemoglobinemia
Hyperkalemia, hyponatremia
Rare: SJS/TEN, anaphylaxis, aseptic meningitis; fever, BUN/creatinine increases

Pregnancy Category C; teratogenic in animals at high doses; may interfere with folate metabolism — use only if potential benefit justifies fetal risk

Lactation Excreted in human milk; caution (folate metabolism interference)

NO BOXED WARNINGS

Principal riskHyperkalemia; raises creatinine without lowering GFR

Cost, est. cash$5–$150/month

Generic entry1985