Frequency of daily over-the-counter drug use and potential clinically significant over-the-counter-prescription drug interactions in the Finnish adult population.

Objective: To explore the frequency of continuous use of over-the-counter (OTC) drugs among the Finnish adult population and the potential for harmful interactions between OTC drugs and prescribed (Rx) drugs.

Methods: Data were extracted from a 1995-1996 population-based interview survey on health care (n = 10,477, response rate 86%). The drug interaction classification system from the Swedish Drug Compendium FASS 1997 was used to identify OTC drugs likely to have clinically significant interactions with prescription drugs. Logistic regression was used to study factors related to continuous use and risks for interactions.

Results: Seventeen percent of the population had used OTC drugs and 15% had used OTC vitamins during the 2 days prior to the interview. Daily use of OTC drugs and of vitamins was reported by 7% and 9%, respectively. Continuous use of OTC drugs was related to older age, female gender, higher education, poor health status, long-term morbidity, psychosomatic symptoms (fatigue) and use of prescription drugs, but not to poor lifestyle. Four percent of the OTC drug users had taken drug combinations with potential for clinically significant interactions. Interactions were most common for ketoprofen (15% of ketoprofen users), ibuprofen (10%), and acetylsalicylic acid (ASA) (6%). The number of prescription drugs, long-term illness and lower education best explained the risk for interactions.

Conclusion: Continuous use and potentially harmful Rx/ OTC drug interactions occur among OTC drug users. Further studies should be done to investigate whether potential combinations will actually lead to clinical problems. The possible interactions of non-steroidal anti-inflammatory drugs (NSAIDs) and analgesics with prescription drugs should especially be taken into account in drug information.