Fosfomycin vs. Keflex

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

Key Takeaways

  • Fosfomycin treats uncomplicated UTI with a single oral dose, while Keflex requires multiple doses a day for 7 to 14 days.

  • Fosfomycin is FDA-approved only for uncomplicated cystitis in women, while Keflex covers many infection types beyond UTI.

  • Keflex carries added warnings around C. difficile diarrhea and seizure risk with kidney impairment that fosfomycin does not share.

Fosfomycin vs. Keflex Drug Summary

Fosfomycin

Fosfomycin is a phosphonic acid derivative antibacterial approved for uncomplicated urinary tract infections (acute cystitis) in women. It comes as an oral granule packet dissolved in water and is taken as a single 3 g dose, rather than a multi-day course.

Keflex

Keflex (cephalexin) is a first-generation cephalosporin antibiotic used for infections like UTIs, skin infections, and respiratory infections. It is taken by mouth as a capsule, tablet, or liquid, typically several times a day for 7 to 14 days.

Fosfomycin vs. Keflex Side Effects

Fosfomycin

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

Keflex

Most common
Diarrhea (rate not quantified in current label)
Nausea (rate not quantified in current label)
Vomiting (rate not quantified in current label)
Dyspepsia (rate not quantified in current label)

Fosfomycin vs. Keflex 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

Keflex

Dosage
Dosage Table
STAGE DOSE WHEN
Starting dose 25-50 mg/kg/day divided At the start
Maximum dose up to 4 g/day divided If needed
Dose changes 3 increases As tolerated

Fosfomycin vs. Keflex Indications

Fosfomycin

Fosfomycin is FDA-approved only for uncomplicated urinary tract infections (acute cystitis) in women caused by susceptible E. coli or E. faecalis. It is not approved for pyelonephritis, complicated UTI, use in men, or children age 12 and under, though doctors sometimes prescribe it off-label for these situations.

  • Acute cystitis in women (FDA-approved)

  • Off-label: complicated UTI or pyelonephritis

  • Off-label: chronic bacterial prostatitis

  • Not approved for men or children 12 and under

Keflex

Keflex is FDA-approved for a broad range of bacterial infections, including respiratory tract infections, ear infections, skin infections, bone infections, and urinary tract infections. It is also used off-label to help prevent infection after certain surgeries.

  • Respiratory tract infections

  • Otitis media (ear infections)

  • Skin and skin structure infections

  • Bone infections

Fosfomycin vs. Keflex Pros and Cons

Fosfomycin

Fosfomycin's single-dose regimen makes it convenient and improves the odds patients complete treatment, but its FDA approval is narrow. It only covers uncomplicated cystitis in women, so more serious or complicated UTIs need a different antibiotic.

Pros

  • Single 3 g dose completes the entire treatment

  • No multi-day schedule of pills to remember

  • Repeat dosing hasn't been shown to work better, so one dose is standard

  • Shorter exposure may mean lower C. diff risk

Cons

  • Only FDA-approved for uncomplicated cystitis in women

  • Not effective for pyelonephritis or complicated UTI

  • Not approved for men or children 12 and under

  • Costs $80 to $145 per dose, more than a typical Keflex course

Keflex

Keflex covers a much wider range of infection types, but treatment requires taking doses multiple times a day for one to two weeks. Its decades of use mean prescribers are very familiar with its dosing and safety profile, though it carries its own risks.

Pros

  • FDA-approved for many infection types beyond UTI

  • Long track record of use since 1971

  • Costs $53 to $95 for a full course, less than one fosfomycin dose

  • Available as capsule, tablet, or liquid suspension

Cons

  • Requires multiple doses a day for 7 to 14 days

  • Risk of C. diff-associated diarrhea with longer courses

  • Seizure risk if it accumulates in kidney impairment

  • Contraindicated in patients allergic to cephalosporins

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