Older adults are vulnerable to adverse drug reactions (ADRs) and drug-drug interactions (DDIs). Evidence on clinically manifest DDIs in older outpatients is scanty. The present study aims to report clinically manifest DDIs, their risk factors, and preventive measures. A subgroup analysis of a 6-year (2015–2021) long prospective study was conducted in a tertiary hospital in North India. Older outpatients with ADRs constituted the study participants. Among 933 ADRs reported in 10,400 patient registrations, clinically manifest DDIs were involved in 199 (21.3%). DDIs accounted for 29.9%, 26.5%, and 21.3% of drug-related metabolic, vascular, and nervous system disorders, respectively. Movement disorders (n = 18), hypotension (n = 16), and hypoglycemia (n = 15) were the most common manifestations. Eighty-six percent of DDIs were of the pharmacodynamic type, and 13.1% were immune-mediated. Around 35% of DDIs resulted in hospitalization, with hyponatremia, movement disorder, and renal impairment as the common reasons. Older adults with Parkinsonism, infection, coronary artery disease, neuropsychiatric disease, and diabetes mellitus, respectively, had 3.28, 2.85, 1.97, 1.76, and 1.80 times higher odds of DDIs. Those receiving ≥ 10 drugs had 5.31 times higher odds of DDIs compared to individuals receiving 1–4 drugs. “Avoiding the causative drug,” “optimal monitoring of the patient,” and “start-low and go-slow” policy together could prevent 85% of DDIs. In conclusion, every fifth case of ADRs and nearly one third of ADR-related hospitalizations in older adults are related to DDIs. Movement disorder, hypotension, and hypoglycemia are the common manifestations. A holistic approach with drug omission, optimal patient monitoring, and slow titration of therapy can prevent significant DDIs in older adults.