Background <p>Acute bronchitis (AB) is a lower respiratory tract infection manifested by cough, which is challenging to manage at the moment. Therefore, this study aimed to evaluate the efficacy and safety of a food supplement (Atusin^® CAP) combining 4 types of purified essential oils, bromelain, and green Brazilian propolis in adults.</p> Methods <p>A post-surveillance, multi-center, randomized, double-blind, placebo-controlled, parallel-group clinical study was conducted over 6 months. Participants with acute uncomplicated bronchitis were administered the combination preparation (verum group) or placebo for 10 days in total. The primary outcome was clinical cure, defined as a ≥ 75% reduction in baseline Bronchitis Severity Scale (BSS) score after 10 days.</p> Results <p>In total, 310 participants were randomized 1:1, of which 155 and 150 participants in the verum and placebo group, respectively, were included in the efficacy and safety analysis sets. A statistically significantly higher number of clinically cured participants was observed in the verum group compared to the placebo group on all reporting days until the end of study intervention administration (Day 10: <i>p</i> = 0.029). The mean BSS score was statistically significantly different between the study intervention groups from Day 7 (<i>p</i> = 0.050) until Day 10 (<i>p</i> = 0.011). In addition, there were no differences in the incidence of adverse events between the verum and placebo group.</p> Conclusions <p>The study shows that the studied combination preparation, containing scientifically proven active substances that bring together key mechanisms for cough relief, is effective in the treatment of acute uncomplicated bronchitis.</p> Trial registration <p>ClinicalTrials.gov identifier: NCT06142994. Registered on 22 November 2023.</p>