Co-trimoxazole (TMP-SMX)

• Pneumocystis jirovecii (PCP) treatment & prophylaxis in HIV
• MRSA SSTI (outpatient)
• Toxoplasmosis prophylaxis (non-pregnant)

Sequential folate blockade: sulfamethoxazole inhibits dihydropteroate synthase, trimethoprim inhibits DHFR — synergistic bactericidal effect.

• T1 — folate antagonism → NTDs
• Term — sulfa displaces bilirubin → kernicterus
• Mid-pregnancy: use only if essential + folate 5 mg

PCP treatment

TMP 15–20 mg/kg/day divided q6–8h × 21 d

UTI (non-preg)

960 mg PO BD × 3 d

PCP prophylaxis

960 mg PO OD or 3×/week

• AbsoluteT1 — folate antagonism (NTDs)
• AbsoluteNear term — kernicterus, methaemoglobinaemia
• AbsoluteG6PD deficiency, sulfa allergy

• Bone marrow suppression
• Stevens-Johnson / TEN
• Hyperkalaemia (TMP blocks ENaC)
• Crystalluria, AKI

• • Avoid T1 and term; if essential mid-pregnancy, co-prescribe folic acid 5 mg/day
• • Standard PCP cover in HIV+ pregnancy with folinic rescue

• Warfarin

  Avoid

  ↑↑ INR

• Methotrexate

  Avoid

  Additive marrow toxicity

• ACEi / ARBs / spironolactone

  Avoid

  Severe hyperkalaemia

• Sulfonylureas

  Caution

  Hypoglycaemia

• 💡 For PCP in HIV+ pregnancy benefit > risk — give with folinic acid
• 💡 Replace with cephalexin or fosfomycin for UTI in pregnancy