Background: Drug interactions increase the risk of treatment failure, intoxication, hospital admissions, consultations and mortality. Computer-assisted prescription systems can help to detect interactions.

Objectives: To describe the drug-drug interaction (DDI) and drug-disease interaction (DdI) prevalence identified by a computer-assisted prescription system in patients with multimorbidity and polypharmacy. Factors associated with clinically relevant interactions were analysed.

Methods: Observational, descriptive, cross-sectional study in primary health care centres was undertaken in Spain. The sample included 593 patients aged 65-74 years with multimorbidity and polypharmacy participating in the MULTIPAP Study, recruited from November 2016 to January 2017. Drug interactions were identified by a computer-assisted prescription system. Descriptive, bivariate, and multivariate analyses with logistic regression models and robust estimators were performed.

Results: Half (50.1% (95% CI 46.1-54.1)) of the patients had at least one relevant DDI and 23.9% (95% CI 18.9-25.6) presented with a DdI. Non-opioid-central nervous system depressant drug combinations and benzodiazepine-opioid drug combinations were the two most common clinically relevant interactions (10.8% and 5.9%, respectively). Factors associated with DDI were the use of more than 10 drugs (OR 11.86; 95% CI 6.92-20.33) and having anxiety/depressive disorder (OR 1.98; 95% CI 1.31-2.98). Protective factors against DDI were hypertension (OR 0.62; 95% CI 0.41-0.94), diabetes (OR 0.57; 95% CI 0.40-0.82), and ischaemic heart disease (OR 0.43; 95% CI 0.25-0.74).

Conclusion: Drug interactions are prevalent in patients aged 65-74 years with multimorbidity and polypharmacy. The clinically relevant DDI frequency is low. The number of prescriptions taken is the most relevant factor associated with presenting a clinically relevant DDI.