EVIDENCE-BASED ANSWER:
It’s unclear, as no studies have specifically evaluated therapies for uncomplicated urinary tract infections (UTIs) in breastfeeding women. However, trimethoprim/sulfamethoxazole (TMP/ SMX), β-lactam antibiotics, nitrofurantoin, and fluoroquinolones all produce cure rates of 78% to 95% for uncomplicated UTIs in women who aren’t breastfeeding, and all appear to be equivalent (strength of recommendation [SOR]: A, a systematic review).
Women who take TMP/SMX develop drug concentrations in breast milk that are below recommended maximum safe levels for infants who don’t have glucose-6-phosphate dehydrogenase (G6PD) deficiency (SOR: B, a small observational study and expert opinion); treatment with nitrofurantoin and ciprofloxacin also produces low levels in breast milk (SOR: C, extrapolations from small observational studies and expert opinion). (Though in the case of nitrofurantoin, this does not include patients with G6PD deficiency.)
Some antibiotics taken by breastfeeding mothers may occasionally be associated with adverse effects in their infants: TMP/SMX may cause poor feeding; amoxicillin and cephalexin may cause diarrhea; nitrofurantoin may cause diarrhea or, in infants with G6PD deficiency, hemolytic anemia; and ciprofloxacin may cause pseudomembranous colitis in infants and green teeth in neonates (SOR: C, case reports and expert opinion).
EVIDENCE SUMMARY
Because no randomized controlled trials have evaluated the efficacy of UTI treatment in lactating women, recommendations are extrapolated from studies in other populations and case reports.
Antibiotics: Comparable and effective
A 2010 Cochrane review examined 21 good-quality randomized trials that compared the effectiveness of TMP-SMX, β-lactam antibiotics, nitrofurantoin, and fluoroquinolones for uncomplicated UTIs in 6016 women.1 The authors found no significant differences in short-term symptom cure rates: all antibiotics were very effective. Seven studies reported mixed (clinical and bacteriologic) cure rates.
Symptom cure rates for patients followed for as long as 2 weeks ranged from 78% to 95%; longer-term (as long as 8 weeks) symptom cure rates ranged from 82% to 91%. The review suggested that TMP-SMX may be slightly more likely to cause a rash than other antibiotics.1
Antibiotic concentrations in breast milk
In a case series, TMP/SMX, 160/800 mg, given to 50 lactating women 2 times (40 women) or 3 times (10 women) daily resulted in an average breast milk concentration of 2 μg/mL of TMP and 4.6 μg/mL of SMX, corresponding to respective doses of 0.3 and 0.7 mg/kg/d for infants taking 150 mL breast milk/kg/d.2 The authors state that this dose is safe for infants without G6PD deficiency. The study included only women with UTIs or other infections requiring antibiotic treatment.
A case series of 4 lactating mothers who received a single 100-mg oral dose of nitrofurantoin found that peak breast milk concentration occurred 4 hours later and averaged 2.4 μg/mL (standard deviation=1.7-3.2 μg/mL).3 The authors calculated a mean concentration over 12 hours of 1.3 μg nitrofurantoin/mL breast milk. This level would correspond to an estimated dose of 0.2 mg/kg/d for an infant consuming 150 mL/kg/d of breast milk whose mother takes 100 mg nitrofurantoin twice daily, much lower than the recommended pediatric dose of 5 to 7 mg/kg/d.