Lactobacillus reuteri, now named Limosilactobacillus reuteri, is a probiotic bacterium. Effects depend on the strain; DSM 17938 and ATCC PTA 6475 are the best studied.
Also known as: L. reuteri, Limosilactobacillus reuteri, DSM 17938, ATCC PTA 6475.
A children's antibiotic diarrhea trial used DSM 17938 at 200 million CFU (Dinleyici 2025). A bone trial in women used ATCC PTA 6475 at 500 million or 5 billion CFU twice a day for 2 years (Gregori 2024). These are study protocols, not a recommendation.
It is sold in the US as a dietary supplement. Under the 1994 dietary supplement law (DSHEA), FDA does not approve dietary supplements before they are marketed, and the maker, not FDA, is responsible for safety and labeling.
What studies show: a 2018 meta-analysis of raw data from 4 trials (345 infants with colic) found strain DSM 17938 cut crying and fussing by about 25 minutes a day more than placebo at day 21 and almost doubled the chance of improvement, but only in breastfed babies; there was not enough data for formula-fed babies (Sung 2018). In 654 children on amoxicillin-clavulanate, DSM 17938 roughly halved antibiotic-associated diarrhea (7.9% versus 16.7% at 14 days) (Dinleyici 2025). In 239 women just past menopause, strain 6475 for 2 years did not slow bone loss (Gregori 2024).
Safety: no significant side effects were seen in the 2-year bone trial (Gregori 2024). The sources here do not describe drug interactions.
Probiotics are live bacteria that act in the gut rather than being absorbed into the blood, so a half-life or time to peak does not apply.
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