How the dose is calculated
Note: the classic adult oral fosfomycin regimen for uncomplicated UTI is a single fixed 3 g sachet, not weight-based. The weight-based figures below apply to paediatric oral dosing (where used) and to the IV formulation used for more severe infections, both divided every 8 hours.
Typical dosing by indication
| Indication | Dose | Frequency | Max |
|---|---|---|---|
| Paediatric oral (UTI) | 40 mg/kg/day | every 8h (3×/day) | 3000 mg/day |
| Severe (IV) | 200 mg/kg/day | every 8h (3×/day) | 16000 mg/day |
Adults are usually dosed with a single fixed 3 g oral sachet for uncomplicated cystitis, not a weight-based regimen — use the paediatric weight-based figures above only where per-kg oral dosing is specifically indicated (e.g. in children), and confirm the product's exact instructions before dispensing.
Clinical use
- Oral fosfomycin's single-dose adult regimen for uncomplicated cystitis is valued for its simplicity and good activity against many resistant Gram-negative organisms, including some ESBL-producing strains.
- IV fosfomycin is used for more severe or multidrug-resistant infections, often as part of a combination regimen rather than monotherapy.
- Not appropriate for pyelonephritis or infections beyond the lower urinary tract when using the standard single-dose oral regimen.
- Each gram of fosfomycin salt carries a significant sodium load — use caution with IV dosing in patients on a sodium-restricted diet or with heart failure.
Frequently asked questions
Is fosfomycin dosed by weight in adults?
Not for the standard oral regimen — adults typically receive a single fixed 3 g sachet for uncomplicated cystitis, regardless of weight. Weight-based dosing applies mainly to paediatric oral use and to the IV formulation.
What is fosfomycin used for?
Oral fosfomycin is used for uncomplicated urinary tract infections (cystitis), including some caused by resistant organisms; IV fosfomycin is used for more severe or multidrug-resistant infections, often in combination with another agent.
Can fosfomycin treat a kidney infection?
The standard single-dose oral regimen is not intended for pyelonephritis or infections beyond the bladder — a different antibiotic or regimen is needed for kidney infections.
Does fosfomycin have any special safety concerns?
IV fosfomycin carries a significant sodium load per gram, so it's used cautiously in patients who need to restrict sodium intake, such as those with heart failure.
References
- Gupta K, et al. International Clinical Practice Guidelines for Uncomplicated Cystitis (IDSA/ESCMID). Clin Infect Dis. 2011.
- British National Formulary for Children (BNFc) — Fosfomycin.
- Falagas ME, et al. Fosfomycin — Use Beyond Urinary Tract Infections. Clin Microbiol Infect. 2019.