Probiotics for Anxiety and Depressive Symptoms in Cancer: A Systematic Review of Animal and Human Studies with Mechanistic Insights

Probiotics have been increasingly evaluated for their potential effect on anxiety and depression through the modulation of the gut-brain axis. Individuals with cancer experience a high prevalence of these symptoms. However, the effects of probiotics and their underlying mechanisms in this population have not been systematically evaluated. This review synthesizes current evidence regarding probiotic interventions for anxiety and depression in cancer and examines the associated mechanistic pathways. A systematic search for original trials in PubMed, Embase, CINAHL, Web of Science, and PsycINFO was conducted in May 2025. Eligible studies included animal models or adults with cancer who received probiotics alone or in combination with other treatments, with outcomes related to anxiety, depressive symptoms, or depression. Search terms included animal model, cancer, probiotics, anxiety, depressive symptoms, depression, gastrointestinal microbiome, gut microbiome, and microbiota. The review followed PRISMA guidelines. Risk of bias in trials was assessed using the SYRCLE and Cochrane RoB2 tool. Nine studies met the inclusion criteria, including seven human studies, one animal study, and one mixed human-animal study, with human sample sizes ranging from 24 to 266. The animal study reported reductions in depressive and anxiety-like behaviors, paralleled by modulation of the hypothalamic-pituitary-adrenal (HPA) axis, reduced inflammation, rebalancing of the gut microbiota, and improvements in neurotransmitter pathways. Findings from human studies were more variable. Some trials reported improvements in anxiety, and depressive symptoms, while others showed no significant differences compared with control groups. Studies that combined probiotics with antidepressants or exercise demonstrated the most pronounced reductions in anxiety and depression. Mechanistic insights from human studies partially aligned with animal evidence, with several trials showing reductions in inflammatory markers (IL-6, TNF-α), improvements in neuroendocrine measures (serotonin, dopamine, cortisol), stabilization of metabolic markers, and favorable shifts in gut microbiota, although these effects were not consistent across all studies. Probiotics appear to be safe within the intervention periods of the reviewed studies (<24 weeks), as no serious adverse effects were reported. Substantial heterogeneity across studies, including variations in cancer type, intervention duration, probiotic strains, formulations, dosages, and study design combined with small sample sizes, restricts the ability to draw definitive conclusions. Rigorously designed randomized controlled trials with larger sample sizes and mechanistic biomarkers are required to confirm the efficacy of probiotics for relieving anxiety and depression in the cancer population.