Abstract Background Adverse drug reactions (ADRs) are common and have serious repercussions for older-adults. This descriptive-analysis elucidates clinical presentations, severity and responsible drugs of incident ADRs in the SENATOR (Software ENgine for the Assessment & optimization of drug and non-drug Therapy in Older peRsons) phase I feasibility study. Methods SENATOR-phase-I was a European multicentre-prospective-observational study. Participants were ≥ 65 years, experiencing acute-hospitalisation and on pharmacological treatment for ≥ 3 conditions. Adverse events (AEs) were identified by trigger list at recruitment, day-14/discharge and classified as ADRs when association with an administered drug was adjudicated as being probable/certain, according to the World Health Organization Uppsala Monitoring Centre ADR causality criteria. Results Of 644 participants recruited, 382 (59.1%) experienced 732 AEs, 363 AEs (49.6%) being incident. 139 participants (21.6%) experienced 177 (48.8%) ADRs. Full drug details were recorded in 156 patients (88%). Cardiovascular-system drugs accounted for one-third of ADRs (55, 35.3%). Five drug classes caused three-quarters of ADRs (135, 76.3%); diuretics 28.2% (furosemide 24.4%), opioid analgesics 16.7%, anti-bacterials for systemic use 14.1%, anti-thrombotic agents 10.3%, and drugs used in diabetes 7%. 68 patients (10.6%) experienced 81 (45.8%) clinically significant moderate-severe ADRs. Significant serum electrolyte disturbance (32, 18.1%), acute kidney injury [AKI] (22, 12.4%), unspecified adverse event (UAE; 21, 11.9%), dyspepsia/nausea/vomiting (20, 11.3%), new-onset major constipation (19, 10.7%) were the most common presentations of ADRs, accounting for 114, (64.4%) of all ADRs. In moderate-severe ADRs; AKI (12, 14.8%), acute bleeding (11, 13.6%), new-onset major constipation (11, 13.6%), UAE (11, 13.6%), significant serum electrolyte disturbance (10, 12.4%) were the most common presentations. Conclusion This analysis highlights that; 1-in-5 older adults will experience an incident ADR during acute hospitalisation. 1-in-10 patients experience moderate-severe incident ADRs. 3-of-4 ADRs were caused by 5 drug classes (diuretics, opioids, anti-bacterials, anti-thrombotics and anti-diabetic agents). This study allows for identification of potentially high risk medications which could be targeted for future ADR prevention.