Information deficits in the summary of product characteristics preclude an optimal management of drug interactions: a comparison with evidence from the literature

Objective: To compare comprehensiveness and accuracy of drug interaction information in the German summary of product characteristics (SPC) with current evidence from the literature and to evaluate the SPC's usefulness with respect to management of drug interactions. Methods: Information on clinically relevant drug interactions was compared between the SPC and three standard information sources on drug interactions (DRUGDEX, Hansten/Horn's Drug Interactions Analysis and Management, Stockley's Drug Interactions) according to five consecutive criteria (inclusion, appropriateness of class labelling, effect description, management recommendation, explicit dose adjustment). Using medication data of an outpatient population (n=4,949), we determined what percentage of insufficiently characterized combinations indeed occurred in outpatients treated with combination drug therapy. Results: Only for 33% (192/579) of the evaluated combinations did SPCs provide drug interaction information equivalent to the evidence from the published literature. Of the clinically relevant drug interactions, 16% were completely missing and 51% were insufficiently characterized compared with standard sources. Explicit management recommendations were either missing or differed from standard sources in 18% of the evaluated pairs of compounds. Of these missing or insufficiently characterized combinations, 12% (47/387) were indeed prescribed to outpatients. Those drug combinations for which the interaction potential was not mentioned in the SPC were received by 0.6% (32/4,949) of patients, and 4% (192/4,949) of patients received combinations that had insufficiently characterized drug interactions. Conclusions: If physicians only rely on SPC information for drug interactions, adverse events due to lacking management recommendations may occur. To meet the SPCs claim of being the basis of information for health professionals on how to use medicinal products safely and effectively, information on drug interactions should be thoroughly up-dated and expanded.