Is fosfomycin safe for use during pregnancy?

Fosfomycin Safety in Pregnancy

Fosfomycin can be used safely during pregnancy, though it should be reserved for situations where clearly needed, as there are no adequate well-controlled studies in pregnant women. 1

FDA Classification and Animal Studies

• The FDA label states fosfomycin should be used during pregnancy "only if clearly needed" due to lack of adequate human studies 1
• Animal reproduction studies show no teratogenic effects in pregnant rats at doses up to 1,000 mg/kg/day (approximately 9 times the human dose) 1
• Fetotoxicities observed in rabbits occurred only at maternally toxic doses and were attributed to intestinal microflora changes, not direct drug toxicity 1
• Fosfomycin crosses the placental barrier in both humans and animals 1

Human Safety Data

Major Congenital Anomalies

• The largest comparative study (2,724 first-trimester exposures) found no increased risk of major congenital anomalies with fosfomycin compared to nitrofurantoin (2.0% vs 2.5%; OR 0.80) or unexposed pregnancies (2.0% vs 2.1%; OR 0.97) 2
• A German prospective cohort study of 152 first-trimester exposures reported only 1 major birth defect (0.7%) versus 3.8% in unexposed controls 3

Pregnancy Outcomes

• Analysis of 5,362 pregnancies showed no significant differences in gestational age, neonatal weight, Apgar scores, or pregnancy complications (urgent cesarean, general anesthesia need, labor induction) between fosfomycin-exposed and unexposed groups 4
• One study reported lower spontaneous abortion rates in fosfomycin-exposed pregnancies (6.2% vs 23.1%; HR 0.35), though this finding requires confirmation 3

Clinical Efficacy in Pregnancy

• Single-dose fosfomycin demonstrates equivalent clinical and microbiological efficacy to other antibiotics for uncomplicated UTI and asymptomatic bacteriuria in pregnant women 5
• Meta-analysis shows comparable outcomes between fosfomycin and comparator antibiotics in pregnant women with UTI or asymptomatic bacteriuria 5
• Fosfomycin maintains urinary concentrations >128 mg/L for 24-48 hours after a single 3-gram dose, adequate to inhibit most uropathogens 6

Important Caveats from Guidelines

• The 2019 IDSA guidelines note that outcomes such as pyelonephritis and preterm labor are not yet well studied for fosfomycin in pregnancy 7
• Nitrofurantoin and β-lactam antimicrobials remain preferred first-line agents due to more extensive safety data in pregnant women 7
• The IDSA specifically identifies the need for studies evaluating fosfomycin's impact on neonatal outcomes 7

Safety Profile

• Most adverse events are gastrointestinal and generally mild 5
• No serious fosfomycin-related adverse events have been reported in pregnancy studies 5
• The single-dose regimen offers superior patient compliance compared to multi-day courses 5, 6

Practical Recommendation

Use fosfomycin as an alternative agent when first-line options (nitrofurantoin, β-lactams) are contraindicated, not tolerated, or when treating multidrug-resistant organisms. The growing body of evidence supports its safety profile, but the limited data on prevention of serious maternal outcomes (pyelonephritis, preterm labor) means it should not routinely replace agents with more extensive pregnancy safety data. 7, 2, 5