In 2013, researchers at UCLA scanned the brains of 36 healthy women before and after a four-week experiment. Some ate a fermented milk product containing probiotic bacteria twice a day, some ate a similar non-fermented milk product and some ate neither. The women who had the probiotic product showed changes in activity in brain regions involved in processing emotion and sensation (Tillisch et al., 2013).
Studies like that helped launch the idea of "psychobiotics," probiotics that might influence mood or stress. More than a decade later, what does the human evidence actually show? The short answer: interesting, but far from settled.
Where the idea came from
Much of the early excitement came from animal research. In rodents, changes in gut bacteria have been reported to alter emotional behavior and how the gut signals pain (Tillisch et al., 2013). In one anxious strain of mice, a specific Lactobacillus rhamnosus strain reduced stress-related behavior and stress hormone release (Kelly et al., 2017).
The brain-scan study was an important step because it looked at people. But it was small, it measured brain activity rather than mood itself, and it was funded in part by a food company whose employees were among the authors (Tillisch et al., 2013). It raised a question more than it answered one.
What happened when a mouse finding was tested in people
Researchers in Ireland gave that same strain, L. rhamnosus JB-1, to 29 healthy men in an 8-week placebo-controlled crossover trial. It didn't beat placebo on mood, anxiety, stress, sleep quality, the stress hormone response, inflammation or thinking skills (Kelly et al., 2017).
The authors titled their paper "Lost in translation?" It's a useful reminder that effects in animals don't automatically carry over to humans. The trial also included only men, so it tells us little about women specifically.
What pooled studies show
A 2018 meta-analysis combined 10 trials with 1,349 participants. Overall, probiotics had no significant effect on mood. When the researchers separated the studies, people with mild to moderate depressive symptoms showed improvement, while healthy people did not. The authors also noted that trials used very different strains and doses, which makes them hard to compare (Ng et al., 2018).
A 2021 meta-analysis focused on people with clinical depression and included 7 studies with 404 participants. Probiotics reduced depressive symptoms when taken alongside antidepressants, but showed no significant benefit when used on their own (Nikolova et al., 2021).
Why might probiotics help as an add-on but not alone? The researchers pointed to possible routes, such as increases in BDNF, a protein involved in brain cell growth, and decreases in CRP, a marker of inflammation. They were careful to add that evidence for these mechanisms is still limited (Nikolova et al., 2021).
A closer look at probiotics as an add-on
The same London research group then ran a small pilot trial. Forty-nine adults with major depression who hadn't fully responded to antidepressants added either a multistrain probiotic or a placebo for 8 weeks. Eighty percent were women, with an average age of about 32 (Nikolova et al., 2023).
The probiotic group showed greater improvement on several, though not all, depression and anxiety scales, and the probiotic was well tolerated. The authors called the results promising and said a larger, definitive trial is needed (Nikolova et al., 2023). It's also worth knowing that this research group received funding from a probiotic manufacturer (Nikolova et al., 2021; Nikolova et al., 2023).
Why the results are so hard to pin down
Part of the problem is that "probiotics" isn't one thing. The trials above used different strains, combinations and doses, in groups ranging from healthy volunteers to people with diagnosed depression (Ng et al., 2018). The individual trials described here were also short, lasting weeks rather than months. Until larger trials test specific strains in clearly defined groups, broad claims about probiotics and mood are ahead of the science.
What this means for you
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Probiotics aren't a substitute for mental health care. The clearest signal so far is as an add-on to standard treatment in people with diagnosed depression, and even that needs larger trials.
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For everyday stress in healthy people, the evidence is weak. Pooled results in healthy volunteers haven't shown a clear mood benefit (Ng et al., 2018).
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Strains aren't interchangeable. Results from one strain or blend may not transfer to another, so look for full strain names when you compare products.
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Judge a probiotic on the reason you take it. If you use one for digestive symptoms, track digestive symptoms and give it a fair trial. Here's what a realistic timeline looks like.
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Don't stop or change prescribed medication to try a probiotic instead.
When to see a doctor
If low mood, anxiety or a loss of interest in things you usually enjoy lasts more than two weeks, or stress is affecting your sleep, appetite or daily life, talk to your doctor or a mental health professional. If you ever have thoughts of harming yourself, call or text 988 in the US to reach the Suicide and Crisis Lifeline. And if you're immunocompromised or seriously ill, ask your doctor before taking any probiotic.
Sources
- Tillisch K, Labus J, Kilpatrick L, et al. Consumption of fermented milk product with probiotic modulates brain activity. Gastroenterology. 2013;144(7):1394-401, 1401.e1-4. PMID 23474283
- Kelly JR, Allen AP, Temko A, et al. Lost in translation? The potential psychobiotic Lactobacillus rhamnosus (JB-1) fails to modulate stress or cognitive performance in healthy male subjects. Brain Behav Immun. 2017;61:50-59. PMID 27865949
- Ng QX, Peters C, Ho CYX, Lim DY, Yeo WS. A meta-analysis of the use of probiotics to alleviate depressive symptoms. J Affect Disord. 2018;228:13-19. PMID 29197739
- Nikolova VL, Cleare AJ, Young AH, Stone JM. Updated review and meta-analysis of probiotics for the treatment of clinical depression: adjunctive vs. stand-alone treatment. J Clin Med. 2021;10(4):647. PMID 33567631
- Nikolova VL, Cleare AJ, Young AH, Stone JM. Acceptability, tolerability, and estimates of putative treatment effects of probiotics as adjunctive treatment in patients with depression: a randomized clinical trial. JAMA Psychiatry. 2023;80(8):842-847. PMID 37314797
This article is for general information and isn't medical advice. Probiotics aren't a substitute for mental health care. Talk to your healthcare provider about low mood or anxiety, and don't stop prescribed medication without medical advice.
