Evaluation of a prescription based record-linkage model for epidemiological studies of long-term adverse effects of drugs--with special regard to combined oral contraceptives.

The Department of Drugs of the Swedish National Board of Health and Welfare undertook a study of the possibilities of a new scheme for post-marketing surveillance by means of prescription and register based epidemiological studies, primarily of combined oral contraceptives (COC). Based on available data on COC usage patterns and incidence rates of the disease at study, it was estimated that study periods, including the necessary time periods for disease development and generation of a sufficient number of cases, would amount to at least 1 to 13 years for cardiovascular outcomes and 8 to 17 years for reproductive cancers. Prospective and unbiased exposure ascertainment would be the most important advantage. However, delay in follow up, the need for extensive individual questionnaire probing and fear of violation of personal integrity could adversely affect the feasibility of the scheme. Chiefly on the grounds of the extended study periods and magnitude of the necessary infrastructure, it was not judged cost-effective to pursue such a scheme for COC exposure only. It was, however, suggested that it would be considered for epidemiological surveillance of other drugs that are commonly used and for which short term and frequent serious side effects are expected, as for instance lipid lowering compounds, beta-blockers, bensodiazepines and other psychotropic drugs.