AimsAdverse drug reactions (ADRs) contribute to poorer patient outcomes and additional burden to the healthcare system. However, data on the true burden, relevant types and drugs causing ADRs are lacking. The aim of this study was to determine the prevalence of ADR‐related hospitalization in the general adult population in Singapore and to investigate their characteristics.MethodsWe prospectively recruited 1000 adult patients with unplanned admission to a large tertiary‐care hospital. Two independent reviewers evaluated all suspected ADRs for causality, type, severity and avoidability. The prevalence of ADR‐related hospitalization was calculated based on ‘definite’ and ‘probable’ ADRs. Logistic regression was used to evaluate predictors for having an ADR at admission.ResultsThe prevalence of all ADRs at admission was 12.4% (95% CI: 10.5–14.6%) and ADRs causing admission was 8.1% (95% CI: 6.5–10.0%). The most common ADRs were gastrointestinal‐related. The most common drug category causing ADRs were cardiovascular drugs. Patients with ADRs had a longer length of stay than those who did not (median 4 vs. 3 days, P = 1.70 × 10−3). About 30% of ADRs at admission were caused by at least one drug with a clinical annotation in the Pharmacogenomics KnowledgeBase (PharmGKB), suggesting that some of these ADRs may have been predicted by pharmacogenetic testing.ConclusionsWe have quantified the burden and characteristics of clinically impactful ADRs in the Singaporean general adult population. Our results will provide vital information for efforts in reducing ADRs through targeted vigilance, patient education and pharmacogenomics in Singapore.