Objective The objective of the study was to evaluate probiotic supplementation effectiveness in reducing antibiotic‐associated diarrhoea (AAD). Method A double‐blind randomised controlled trial (registration number: ACTRN 12609000429257); with primary outcome prevention of AAD and secondary outcome reduction in diarrhoea duration, patients were randomised to receive probiotic supplementation with Lactobacillus casei, Shirota strain or placebo. Results Eighty‐seven people (66–101 years old) were recruited to the study during their admission to the Geriatric Evaluation Management wards during August–October 2009. 41 received the active Yakult (Yakult Australia Pty Ltd, Dandenong South, Victoria, Australia) (12.2% diarrhoea) and 46 received placebo (8.7% diarrhoea). There was no significant difference in diarrhoea between the groups, Fisher's exact test, P = 0.729. The mean diarrhoea duration was 4 and 5 days for those administered Yakult and placebo, respectively. Conclusion There was no significant difference between groups. However, we noted a low prevalence of AAD and this affects the sample size that will be required for any future study targeting a beneficial effect of probiotic supplementation.