<p>Primary dysmenorrhea is painful menstruation in the absence of pelvic pathology, whereas secondary dysmenorrhea is menstrual pain attributable to an underlying pelvic disease (e.g., endometriosis). Nonsteroidal anti-inflammatory drugs (NSAIDs) are commonly used as first-line therapy. Recent evidence ¹⁸ suggests that modulation of the gut microbiota may influence menstrual pain through immunologic and neuroendocrine pathways. This double-blinded, randomized, placebo-controlled trial investigated the efficacy of daily multispecies probiotic supplementation in reducing menstrual pain severity in women aged 18–24&#xa0;years diagnosed with moderate to severe primary dysmenorrhea. Forty-eight participants were randomized to receive either a probiotic supplement or placebo for three consecutive menstrual cycles, followed by a three-month observation period. The primary outcome was the change in pain severity, assessed using a 10&#xa0;cm visual analog scale (VAS). At baseline, VAS scores were similar between groups (probiotic 6.1 ± 1.17 vs placebo 6.3 ± 1.26; p = 0.62). After three months of intervention, the probiotic group demonstrated a significantly greater reduction in pain scores compared to the placebo group (3.7 ± 1.84 vs 5.8 ± 2.14; p &lt; 0.01). However, the effect was not sustained after discontinuation of supplementation. No serious adverse events were reported. These findings suggest that continuous intake of multispecies probiotics may be an effective non-hormonal adjunct therapy for primary dysmenorrhea. Clinical trial registration: Thai Clinical Trials Registry (TCTR20230326001), registered on 22 March 2023. <a href="https://thaiclinicaltrials.org/">https://thaiclinicaltrials.org/</a></p>