Awareness related to the risk/benefit profile of therapies used in paediatric and elderly patients is limited. We carried out a study, called the MEAP 3.0 study, to collect and analyse evidence of adverse drug reactions (ADRs) and drug–drug interactions (DDIs) that occurred in frail populations under polypharmacy in a real-world setting. Data were retrieved from reports of ADRs and pharmacological counselling from patients treated in hospitals and territorial health services. We collected 2977 ADRs reports and identified ‘anti-infectives for systemic use’ and ‘cardiovascular system’ as the most frequently implicated pharmacological classes in under-18 and over-65 patients, respectively. We detected 2179 DDIs, of which 10.7% were related to at least one ADR: 22 were classified as ‘contraindicated’ (7 in the paediatric group and 15 in the elderly one), and 61 as ‘major’ (6 in the paediatric patients and 55 in the geriatric ones), while 151 DDIs were classified as ‘moderate’ (10 referred to paediatric population, and 109 to elderly patient) and as ‘minor’ (1 in paediatric patients, and 31 in the elderly ones). The MEAP 3.0 project demonstrates that pharmacovigilance surveillance and therapeutic reconciliation are valid strategies to avoid potential DDIs and the occurrence of ADRs, allowing for personalised medicine.