Antibiotic-associated diarrhea (AAD) is a common clinical complication resulting from antibiotic-induced gut microbiota dysbiosis. LA85, a probiotic strain illustrated with in vitro antimicrobial and immunomodulatory properties, may offer a preventive approach against AAD. However, clinical evidence on its efficacy remains limited. This randomized, double-blind, placebo-controlled trial evaluated the efficacy and safety of LA85 in preventing amoxicillin-associated diarrhea. A total of 82 adult participants receiving amoxicillin treatment were randomized to receive either LA85 (2 × 10 CFU/day) or placebo for 14 days. The primary outcomes included AAD incidence, diarrhea duration, and stool consistency, while secondary outcomes assessed gastrointestinal quality of life and safety. LA85 supplementation was associated with a trend toward a reduction in the incidence of AAD; however, this difference did not reach statistical significance. Nonetheless, LA85 notably shortened the duration of diarrhea episodes compared to placebo ( = 0.072), suggesting a clinically meaningful improvement. Participants receiving LA85 exhibited less variability in stool consistency scores, assessed by the Bristol stool form scale (BSFS), maintaining scores consistently around 3.5. In contrast, placebo recipients had greater fluctuations between 3.5 and 4.0, indicating less stable stool consistency during antibiotic treatment. Importantly, exploratory subgroup analysis revealed that in younger participants (< 53 years age), LA85 supplementation significantly reduced the incidence of AAD ( = 0.008) and effectively eliminated persistent diarrhea episodes. Gastrointestinal quality of life scores improved significantly in the probiotic group ( < 0.05). No serious adverse events were reported, supporting the safety of LA85. While these findings support the clinical application of LA85 for preventing AAD, further large-scale trials incorporating microbiome analysis and longer follow-up periods are necessary to confirm its long-term benefits and generalizability. This clinical trial (ClinicalTrials.gov Identifier: NCT05974657) was registered on August 3, 2023.