[Interaction of prescribed drugs in a population over 65 years of age]

To assess the prevalence and the types of drug to drug interactions in a population 65 years and over and to identify the factors associated to the occurrence of interactions.

Cross sectional study ("Aging in Leganés"), a community survey.

Community level. Leganés (Madrid).

People 65 years and over (n = 1284, response rate 83%).

Information on use of health services and medication use was collected through home interviews. Drugs were classified by its active components and into therapeutic categories. Drug to drug interactions were identified using Hansten's classification. Bivariate and multivariate analysis were carried out to identify factors associated to the frequency of interactions.

The prevalence of drug to drug interactions was estimated as 13.6%. Interactions of clinical significance occurred in 9.9% of the population 65 and over. Hypertension, diabetes, Parkinson and cardiovascular diseases are significatively associated to interactions. The number of visits to the specialists was a risk factor (OR = 2.1 for those with five or more visits), after controlling for specific chronic conditions. Contacts with primary care, age and sex of the elderly were not associated to the frequency of interactions.

The frequency of interactions is associated to preventable factors. Improvements in the communication between specialists and primary care and cooperation with pharmacists could result in better monitoring of medication use in the elderly.