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Prevalence and predictors of potential drug–drug interactions in Regione Emilia-Romagna, Italy


  • This work was presented in part at the International Society for Pharmacoeconomics and Outcomes Research (ISPOR) 12th Annual International Meeting, Arlington, VA, USA, May 2007.

Vittorio Maio, PharmD, MS, MSPH, Department of Health Policy, Jefferson Medical College, 1015 Walnut Street, Suite 115, Philadelphia, PA 19107, USA. Tel.: 215 955 1821; fax: 215 923 7583; e-mail:


Background and objective:  Drug–drug interactions (DDIs) are preventable medication errors associated with potentially serious adverse events and death. Several studies have examined the prevalence of potential DDIs among ambulatory patients in various countries. Limited recent data on the prevalence of potential DDIs in Italy are available in the medical literature. The objective of this study was to estimate the prevalence of clinically important potential DDIs among the approximately 4 million residents of Regione Emilia-Romagna (RER), Italy, and to examine possible predictors of potential DDI exposure.

Methods:  A retrospective follow-up study of 2004 outpatient prescription data from RER was conducted. A previously published list of clinically important potential DDIs was refined to include only pairs of drugs in which both drugs were reimbursed by the 2004 Italian National Formulary. A potential DDI was defined as the presence of a minimum 5-day overlap in days supply for drugs in an interacting pair. The 1-year period prevalence of each potential DDI was quantified. A logistic regression analysis was conducted to examine patient characteristics as predictors of potential DDIs.

Results and discussion:  The list of clinically important potential DDIs included 12 drug pairs that could be captured using the RER database. These 12 potential DDIs occurred 8894 times in the RER population in 2004. The most commonly identified potentially interacting medication pairs were warfarin and non-steroidal anti-inflammatory drugs (6824 cases), theophylline/aminophylline and ciprofloxacin/fluvoxamine (930), and warfarin and barbiturates (567). Odds of exposure were highest among those aged 65 years or older, males, and those with more chronic conditions. Odds of exposure increased 1·39 times with each addition of a prescription medication.

Conclusion:  A substantial number of clinically important potential DDIs were identified, particularly among warfarin users. Awareness of the most prevalent potential DDIs can help practitioners prevent concomitant use of these dangerous medication combinations.