Aim: To compare the effectiveness of amoxicillin‐clavulanate versus amoxicillin for adults diagnosed with acute sinusitis (AS). Material and Methods: Adults with AS who were administered amoxicillin and clavulanate underwent a retrospective cohort study. Return visits for sinusitis associated with amoxicillin versus amoxicillin-clavulanate were the main outcome. Hospitalisations, gastrointestinal-related adverse events (AEs), and 30-day infectious complications were all secondary outcomes. Models using logistic regression and propensity score matching were adjusted for relevant confounders. Results: There were 100 AS patient visits total, of which 29 had amoxicillin and 71 had amoxicillin and clavulanate prescribed. 75, 75% of the patients were men. There was no difference between amoxicillin (7%) and amoxicillin/clavulanate (8%), in terms of repeat visits due to sinusitis. Amoxicillin had a lower rate of gastrointestinal adverse events (AEs) than amoxicillin and clavulanate (2%). Conclusion: With less gastrointestinal side effects, amoxicillin showed equivalent efficacy to amoxicillin/clavulanate for AS. When an adult with AS meets the criteria for antibiotic therapy, amoxicillin is a good option. Keywords: adverse drug event, amoxicillin‐clavulanate, antimicrobial stewardship, clinical outcomes, outpatient, sinusitis.