How the dose is calculated
Co-trimoxazole (trimethoprim-sulfamethoxazole) is dosed by its trimethoprim (TMP) component in mg/kg/day; the sulfamethoxazole component follows automatically at the product's fixed 1:5 ratio. Dosing differs substantially by indication — routine infection, active Pneumocystis jirovecii pneumonia (PCP) treatment, and PCP prophylaxis each use very different total daily doses.
Typical dosing by indication
| Indication | Dose | Frequency | Max |
|---|---|---|---|
| Standard infection | 8 mg/kg/day (TMP) | every 12h (2×/day) | 320 mg/day |
| PCP treatment | 20 mg/kg/day (TMP) | every 6h (4×/day) | 1600 mg/day |
| PCP prophylaxis | 5 mg/kg/day (TMP) | once daily | 160 mg/day |
All doses above refer to the trimethoprim (TMP) component only — the sulfamethoxazole dose follows automatically at the standard fixed 1:5 ratio (e.g. 40 mg TMP + 200 mg SMX per 5 mL). Confirm the product's exact ratio before dispensing.
Clinical use
- Co-trimoxazole is used for uncomplicated UTI, some respiratory infections, and at much higher doses for treating active Pneumocystis jirovecii pneumonia (PCP) in immunocompromised patients.
- PCP prophylaxis uses a far lower dose than PCP treatment — mixing these up is a well-recognised, dangerous prescribing error.
- Contraindicated in known sulfonamide allergy, and used cautiously in glucose-6-phosphate dehydrogenase (G6PD) deficiency (haemolysis risk) and significant renal or hepatic impairment.
- Can cause hyperkalaemia, especially at PCP-treatment doses or in renal impairment — monitor electrolytes during high-dose or prolonged therapy.
Frequently asked questions
Why is co-trimoxazole dosed by the trimethoprim component?
Trimethoprim is the component whose dose is titrated to the indication being treated — the sulfamethoxazole dose follows automatically because the two are combined in a fixed ratio within each product.
Why is the PCP treatment dose so much higher than the prophylaxis dose?
Treating active Pneumocystis jirovecii pneumonia requires a much higher trimethoprim dose (around 20 mg/kg/day) than preventing it in an at-risk but currently uninfected patient (around 5 mg/kg/day) — confusing the two doses is a recognised and dangerous prescribing error.
Is co-trimoxazole safe in G6PD deficiency?
It requires caution — sulfonamides, including the sulfamethoxazole component, can trigger haemolysis in patients with glucose-6-phosphate dehydrogenase (G6PD) deficiency. Confirm G6PD status where relevant before prescribing.
Does co-trimoxazole affect blood potassium levels?
Yes — it can cause hyperkalaemia, particularly at the higher doses used for PCP treatment or in patients with renal impairment, so electrolyte monitoring is advised during high-dose or prolonged courses.
References
- Centers for Disease Control and Prevention. Guidelines for the Prevention and Treatment of Opportunistic Infections.
- British National Formulary for Children (BNFc) — Co-trimoxazole.
- American Academy of Pediatrics Red Book — Pneumocystis jirovecii Infections.