Recurrent urinary tract infections are a problem for many postmenopausal women. Estrogen replacement restores atrophic mucosa, lowers vaginal pH, and may prevent urinary tract infections.

We enrolled 93 postmenopausal women with a history of recurrent urinary tract infections in a randomized, double-blind, placebo-controlled trial of a topically applied intravaginal estriol cream. Midstream urine cultures were obtained at enrollment, monthly for eight months, and whenever urinary symptoms occurred. Vaginal cultures and pH measurements were obtained at entry and after one and eight months. The women were assigned to receive either estriol (n = 50) or placebo (n = 43), both administered intravaginally; 36 and 24, respectively, completed the eight months of follow-up.

The incidence of urinary tract infection in the group given estriol was significantly reduced as compared with that in the group given placebo (0.5 vs. 5.9 episodes per patient-year, P<0.001). Survival analysis showed that more of the women in the estriol group than in the placebo group remained free of urinary tract infection (P<0.001). Lactobacilli were absent in all vaginal cultures before treatment and reappeared after one month in 22 of 36 estriol-treated women (61 percent) but in none of the 24 placebo recipients (P<0.001). With estriol the mean vaginal pH declined from 5.5 to 3.8 (P<0.001), whereas there was no significant change with placebo. The rate of vaginal colonization with Enterobacteriaceae fell from 67 percent to 31 percent in estriol recipients but was virtually unchanged (from 67 to 63 percent) in the placebo recipients (P<0.005). Side effects were minor, but caused 10 estriol recipients (28 percent) and 4 placebo recipients (17 percent) to discontinue treatment.

The intravaginal administration of estriol prevents recurrent urinary tract infections in postmenopausal women, probably by modifying the vaginal flora.