Healthcare workers face substantial occupational demands and exposure to respiratory infections during winter, but evidence that probiotics improve their health and wellbeing remains limited. This exploratory, double-blind, randomised, placebo-controlled trial assessed whether daily Lab4 supplementation affected upper respiratory tract infection (URTI) frequency and supported wellbeing in 104 healthcare workers. Daily diaries recorded symptoms defining URTI episodes, general-practitioner visits, illness-related work absence and antibiotic use. Wellbeing was assessed at baseline and weeks 8 and 16, and faecal microbiota were assessed using 16S rRNA gene sequencing. Participants receiving Lab4 reported better wellbeing than those receiving placebo. For probiotic versus placebo, the risk ratio (RR) for URTI occurrence was 0.60 (95% confidence interval [CI]: 0.30-1.20; p=0.1477), the incidence rate ratio (IRR) for episodes was 0.58 (95% CI: 0.27-1.23; p=0.1581), and the mean ratio for episode duration was 0.98 (95% CI: 0.72-1.32; p=0.8776). For general-practitioner visits, the RR for at least one visit was 0.48 (95% CI: 0.21-1.07; p=0.0737) and the IRR was 0.44 (95% CI: 0.18-1.06; p=0.0677). For illness-related work absence, the RR for at least one day absent was 0.75 (95% CI: 0.18-3.22; p=0.6991) and the IRR for absence days was 0.74 (95% CI: 0.16-3.32; p=0.6914). For antibiotic use, the RR for any use was 0.48 (95% CI: 0.20-1.18; p=0.1111) and the IRR for antibiotic-use days was 0.42 (95% CI: 0.16-1.12; p=0.0820). No significant between-group differences in gut microbiota were detected in the subset providing faecal samples. Illness-related estimates generally favoured Lab4, although between-group differences were not statistically significant. These exploratory findings support evaluation in a larger, prospectively powered trial.