Swallowing a probiotic rarely fixes a UTI. Most of the capsule is dissolved by stomach acid; only a few survivors reach the far end of the gut, and almost none reach the vaginal tissue where UTIs are actually prevented.

In a head-to-head trial, the vaginal route beat the pill. Over 4 months, UTIs recurred in 70% (placebo), 61% (oral), 41% (vaginal), and 32% (combined) of women — Gupta et al., Clin Infect Dis 2024.

Colonization is the whole game. The LACTIN-V trial (vaginal L. crispatus CTV-05) was only modest on average, but in the women whose bodies actually took up the strain, UTIs fell by 93% (Stapleton et al., 2011).

After menopause, you have to feed the bacteria first. When estrogen drops, the vaginal wall thins and its glycogen “food” disappears, so Lactobacillus starves and vaginal estrogen restores the food supply for healthy bacteria.

Match the strain, the route, and the substrate. The 80/20 rule here is four words: strain, route, substrate, colonization. Miss one and the probiotic does nothing.