Fosfomycin vs. Trimethoprim

See how Fosfomycin and Trimethoprim compare on approved uses, dosing, side effects, and cost.

Key Takeaways

  • Fosfomycin treats uncomplicated UTIs with one 3 g dose, while trimethoprim requires a 10-day course of twice- or once-daily tablets.

  • Both drugs are FDA-approved only for uncomplicated cystitis in adults and should not be used for pyelonephritis or complicated UTI.

  • Trimethoprim carries risks not shared by fosfomycin, including hyperkalemia, blood cell disorders, and severe skin reactions like Stevens-Johnson syndrome.

Fosfomycin vs. Trimethoprim Drug Summary

Fosfomycin

Fosfomycin is a phosphonic acid derivative antibacterial used for uncomplicated urinary tract infections in women. It comes as oral granules mixed with water and taken as a single 3 g dose. Repeating the dose has not been shown to work better and raises the chance of side effects.

Trimethoprim

Trimethoprim is a urinary anti-infective that blocks bacterial folate production to treat acute uncomplicated UTIs. It is taken as a 100 mg tablet every 12 hours or a 200 mg tablet once daily for 10 days. Because it does not contain a sulfonamide, it can be used in some patients with sulfa allergies.

Fosfomycin vs. Trimethoprim Side Effects

Fosfomycin

Most common
Diarrhea (up to 10.4%)
Headache (up to 10.3%)
Vaginitis (up to 7.6%)
Nausea (up to 5.2%)

Trimethoprim

Most common
Rash (2.9% to 6.7%)
Nausea (common, rate not specified)
Vomiting (common, rate not specified)
Glossitis (common, rate not specified)

Fosfomycin vs. Trimethoprim Dosage

Fosfomycin

Dosage
Dosage Table
STAGE DOSE WHEN
Starting dose 3 g single dose At the start
Maximum dose 3 g single dose Same as starting
Dose changes None Fixed dose

Trimethoprim

Dosage
Dosage Table
STAGE DOSE WHEN
Starting dose 100 mg every 12 hours Days 1-10
Maximum dose 200 mg once daily Days 1-10
Dose changes 1 increase As tolerated

Fosfomycin vs. Trimethoprim Indications

Fosfomycin

Fosfomycin is FDA-approved for uncomplicated UTIs (acute cystitis) in women caused by susceptible E. coli or E. faecalis. It is not approved for men, children 12 and under, or pyelonephritis. Doctors sometimes use it off-label, in repeated doses, for complicated UTI or chronic prostatitis.

  • FDA-approved for uncomplicated cystitis in women

  • Off-label for complicated UTI or pyelonephritis (multi-dose)

  • Off-label for chronic bacterial prostatitis

  • Not approved for men or children 12 and under

Trimethoprim

Trimethoprim is FDA-approved for acute uncomplicated UTIs caused by susceptible E. coli, Proteus, Klebsiella, Enterobacter, or coagulase-negative staphylococci. It is not approved for infants under 2 months old. Off-label, it is used to prevent recurrent UTIs and, combined with dapsone, to prevent Pneumocystis pneumonia.

  • FDA-approved for acute uncomplicated UTIs

  • Off-label for recurrent UTI prevention

  • Off-label PCP prophylaxis with dapsone

  • Not approved for infants under 2 months

Fosfomycin vs. Trimethoprim Pros and Cons

Fosfomycin

Fosfomycin's single-dose regimen makes it a convenient option when finishing a multi-day antibiotic course is a concern. It costs more per treatment than generic trimethoprim and is not an option for men or complicated infections.

Pros

  • Single 3 g dose completes the full treatment

  • No multi-day dosing schedule to remember

  • Effective against some E. coli resistant to other antibiotics

  • Fewer drug interaction concerns than many antibiotics

Cons

  • Not approved for pyelonephritis or complicated UTI

  • Not approved for men or children 12 and under

  • Costs more per treatment, roughly $80 to $145 per dose versus $45 to $75 for trimethoprim's full course

  • Higher reported rates of diarrhea and headache

Trimethoprim

Trimethoprim has decades of clinical use and a lower cost, but requires a full 10-day course to complete treatment. It carries warnings for hyperkalemia, blood disorders, and severe skin reactions that are not associated with fosfomycin.

Pros

  • Lower cost, roughly $45 to $75 for a full 10-day course versus $80 to $145 for fosfomycin

  • Option for patients with sulfa allergy since it lacks a sulfonamide

  • Flexible dosing: once or twice daily options

  • Long track record of use since 1980

Cons

  • Requires a full 10-day course rather than one dose

  • Risk of hyperkalemia, including in patients without risk factors

  • Risk of blood disorders like megaloblastic anemia

  • Risk of severe skin reactions including Stevens-Johnson syndrome

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