Lactococcus rhamnosus (commonly referred to as Lactobacillus rhamnosus, especially in older literature) is a probiotic bacterium that has been studied for its potential effects on mood and anxiety. The scientific interest stems from the "gut-brain axis" — the bidirectional communication between the gastrointestinal tract and the central nervous system. Some preclinical studies, particularly in rodents, have demonstrated that certain strains of L. rhamnosus can reduce anxiety-like behaviors, possibly by modulating GABA receptor expression and influencing stress hormone levels. For example, a widely cited 2011 study by Bravo et al. in mice showed anxiolytic effects and changes in GABA signaling after administration of L. rhamnosus JB-1.

However, direct evidence in humans is limited. A few small clinical trials have explored the effects of L. rhamnosus supplementation on mood and anxiety symptoms, with mixed results. Some studies report modest improvements in self-reported anxiety, while others find little to no effect. Meta-analyses of probiotics and anxiety often group multiple bacterial strains and report heterogeneity in outcomes, making it difficult to attribute effects specifically to L. rhamnosus.

In summary, while there is a scientific rationale and some preclinical evidence for the use of L. rhamnosus in supporting anxiety disorders, robust clinical validation in humans is lacking. The current evidence base is preliminary and does not yet support routine clinical use for anxiety disorders.