Prevalence and patient awareness of selected potential drug interactions with self-medication

J Clin Pharm Ther. 2007 Apr;32(2):149-59. doi: 10.1111/j.1365-2710.2007.00809.x.


Background and objective: In community pharmacies potential drug interactions between prescription only medicines (POM) and over-the-counter (OTC) drugs purchased for self-medication arise mainly in two situations: (i) if an OTC drug is purchased by a passer-by customer whose prescribed drug therapy is not known; or (ii) if a POM or an OTC drug is requested by a regular customer whose prescribed drug therapy is usually recorded. With this study we aimed to assess the prevalence of potential drug interactions with selected POM and OTC drugs in passer-by and regular customers as well as their awareness of these potential drug interactions.

Methods: Data were collected in 14 community pharmacies in the region of Basel, Switzerland by observation of customer contacts and interviews with passer-by customers purchasing selected OTC drugs, and telephone-interviews with regular customers treated with selected POMs identified in community pharmacies' databases. The selected POMs and OTC drugs are drugs which could lead to clinically relevant drug interactions of varying severity but manageable through different interventions such as adjustment of dose and its timing and/or monitoring of the therapy, and avoidance of the combination by choosing an alternative treatment.

Results: Of 1183 passer-by customers observed, 164 (14 x 4%) purchased at least one of the selected OTC drugs. One hundred and two (62 x 2%) of those subjects were interviewed. Forty-three (42 x 2%) mentioned taking prescribed drugs, and three of them were exposed to potential drug interactions of moderate severity. Out of 592 regular customers selected from the community pharmacy database, 434 (73 x 3%) could be interviewed. Sixty-nine (15 x 9%) of them were exposed to a potential drug interaction between purchased OTC drug for self-medication and their POM. Furthermore, 116 (26 x 7%) regular customers were exposed to potential drug interactions within their prescribed drugs and in 28 (6 x 5%) multiple (>or=2) potential drug interactions were found. Two hundred and three (46 x 8%) regular customers were aware of potential drug interactions between their POM and OTC drugs. Ninety-six (47 x 3%) of them were informed by their prescribing physician and 52 (25 x 6%) by their community pharmacist. Awareness of potential drug interaction was higher in younger customers [odds ratio (OR) 0 x 95; 95% confidence intervals (CI) 0 x 93, 0 x 97, P<0 x 0001] and higher for drug interactions classified as 'severe' [OR 1 x 79; 95% CI 1 x 16, 2 x 77, P=0 x 009].

Conclusion: Efforts to increase awareness of potential drug interactions is needed. Although community pharmacies are adequately equipped with computerized drug interaction surveillance systems this is often not applied to self-medication. Vigilance for potential interactions of all drugs, including those sold over the counter, should be increased.

MeSH terms

  • Administration, Oral
  • Adult
  • Aged
  • Anticoagulants / administration & dosage
  • Anticoagulants / adverse effects
  • Aspirin / administration & dosage
  • Aspirin / adverse effects
  • Clinical Pharmacy Information Systems / statistics & numerical data
  • Dose-Response Relationship, Drug
  • Drug Interactions*
  • Drug-Related Side Effects and Adverse Reactions*
  • Female
  • Humans
  • Immunosuppressive Agents / administration & dosage
  • Immunosuppressive Agents / adverse effects
  • Interviews as Topic / methods
  • Male
  • Middle Aged
  • Monoamine Oxidase Inhibitors / administration & dosage
  • Monoamine Oxidase Inhibitors / adverse effects
  • Patient Education as Topic*
  • Pharmaceutical Preparations / administration & dosage
  • Pharmacies / statistics & numerical data
  • Pharmacists
  • Professional Role
  • Self Medication*
  • Sodium Channel Blockers / administration & dosage
  • Sodium Channel Blockers / adverse effects
  • Tetracyclines / administration & dosage
  • Tetracyclines / adverse effects


  • Anticoagulants
  • Immunosuppressive Agents
  • Monoamine Oxidase Inhibitors
  • Pharmaceutical Preparations
  • Sodium Channel Blockers
  • Tetracyclines
  • Aspirin