Potential drug--drug interactions in 5,125 mostly elderly out-patients in Gothenburg, Sweden.

Of 5,125 mostly elderly out-patients, average age 78.2 years, 1,594 (31%) had at least one interacting drug combination according to the Swedish National Formulary. On average, each patient with drug-drug interactions (DDIs) had 1.6 DDIs. There was no difference in the sex or age between those having DDIs and those without. However, patients with one or more DDIs used a significantly larger number of drugs than those without DDIs, on average 8.1 versus 5.2. This significant difference was independent of age and sex. When classified for clinical significance according to the handbook "Drug Interactions and Updates" by Hansten and Horn, 155 (3%) patients had interactions of 'major clinical significance'. The most common were interactions between beta-adrenergic blockers and antidiabetics, followed by potassium-sparing diuretics and potassium, and carbamazepine and dextropropoxyphene. The methodology allows us to screen routinely for DDIs and to plan further studies with emphasis on clinical outcome.