Stressed teenagers took a probiotic for 12 weeks. Their stress scores fell, and so did the scores of the teens on placebo

Probiotics are often pitched at stress, on the idea that gut bacteria talk to the brain and to the immune system. Most studies have been in adults, so a team at the University of Utah tried one in younger people. Their study, run at Primary Children's Hospital with pupils recruited through Salt Lake and Jordan school districts, was published in the journal Pharmaceuticals on 21 October.

Of 223 people who came forward, 110 were eligible, meaning moderate stress on a standard questionnaire and no SSRI antidepressants. 104 were randomised and 87 finished: 45 on the probiotic and 42 on placebo, average age about 15, nearly all white. For 12 weeks they took capsules daily, preferably with dinner. Each active capsule held 2 billion colony-forming units of three strains, two of Lactiplantibacillus plantarum and one of Levilactobacillus brevis. Only the participants were blinded. Stress was scored at the start, at 8 weeks and at 12 weeks, and participants collected morning saliva at home for cortisol and immune markers.

Stress scores fell in both groups, and the differences between the groups were not statistically significant. The authors suggest pupils may simply have adapted to stress over the school year. Morning cortisol rose in both: by a median 20% against 56% at 8 weeks, and 3% against 7% at 12 weeks, probiotic against placebo. Those gaps were not significant either, yet the paper concludes that the probiotic "slowed the rise". The rise reached significance within the placebo group and not within the probiotic group, but that is not the same as the two groups differing. Of seven saliva immune markers, one did differ between groups: IL-1β, an inflammatory signal (p = .03 at 8 weeks, .004 at 12 weeks). Even there, the 2% and 5.5% falls in the probiotic group were not significant on their own, while the placebo group's level rose 6 to 7%. On our reading, the gap owes at least as much to placebo drifting up as to the capsule pushing down.

The limits are large, and the authors state several. This was a feasibility study, sized to get more than 30 finishers per group rather than to detect an effect, and they call for a larger, more diverse trial. Nothing was recorded about early-life adversity or what was stressing these teenagers at the time. The results describe the 87 who finished, with no intention-to-treat analysis described. Kaneka Americas Holding funded the work and Kaneka North America supplied the capsules, though the authors declare no conflicts of interest. The registry record, NCT07565883, was first submitted on 15 April 2026, for a study that began in May 2022 and was completed in November 2024, so it cannot show what was planned beforehand. It lists emotional and cognitive tests as the primary outcome and morning and evening cortisol as secondary; the paper calls stress and cortisol primary and reports morning cortisol only, with no cognitive results.