Prescribers' Knowledge of Drug Interactions

Drs. Horn and Hansten are both professorsof pharmacy at the Universityof Washington School of Pharmacy.For an electronic version of this article,including references if any, visitwww.hanstenandhorn.com.

The ability of prescribers to accuratelyidentify and manage potentialdrug?drug interactions hasnot been well defined. At least 2 studieshave attempted to assess prescriberknowledge about drug interactions.^1,2The first study reported on physicians,nurse practitioners, and physician assistants(n = 168) in the California VeteransAffairs health system. The second reportincludeda sample drawn from the sametypes of practitioners (n = 950) butincluded all parts of the United Statesand non?Veterans Affairs prescribers.Both studies listed drug pairs and askedthe prescribers to identify those thatare "contraindicated," "may be usedwith monitoring," or that representednoninteracting drug pairs. A majority ofrespondents in both studies reportedthat their knowledge of drug interactionsaffected their ability to select safe drugcombinations.

The Table lists the interacting drugpairs and the percentage of respondentswho either did not think an interactionwas possible or simply did not know ifthe drug pair interacted. It is of interestthat one of the oldest known interactions(warfarin?cimetidine) and mostrecently described interactions (sildenafil?isosorbide in study 2) were relativelywell known to prescribers.

A majority of respondents were notaware of the interactions involving theobject drugs alprazolam, cyclosporine,and methotrexate. Each of these interactionscan produce severe adversepatient outcomes. Perhaps the relativeinfrequency of prescribing methotrexateor cyclosporine by general practitionerscontributes to their lack of knowledge.Unfortunately, both of these studiesdemonstrate a poor understanding ofdrug interactions among the sampledpractitioners. Of the 12 separate drugcombinations tested, only one was recognizedcorrectly by &gt80% of the respondents.Due to the limited number ofinteractions assessed in these studies,neither can be considered a definitiveassessment of prescribers' knowledgeabout drug interactions; however, theresults do not provide much assurancethat prescribers are well-equipped tointegrate drug interaction potential intotheir therapeutic decision process.

Both studies noted that prescribersturned to outside resources for assistancewith potential drug interactions.Pharmacists were identified as the mostcommon source that identified a potentialinteraction, and the prescribers weremore likely to respond to a pharmacist'scontact than one originated from analternative source. Information regardingdrug interactions was often reported tobe associated with changes in prescribingchoices.

Although a majority of practitionersunderstand the importance of recognizingpotential drug interactions, they do notappear to have the knowledge to achievethe desired outcome. Fortunately, theyare quite willing to be assisted in theidentification of potential drug interactions.Pharmacists should not assumea prescriber knows of a potential interactionand has done a complete risk?benefit assessment prior to ordering thedrugs. Even if the prescriber is awareof the interaction, his or her knowledgemay be limited to a few words in thedrug labeling. Providing a brief overviewof the potential interaction and methodsof avoiding patient harm will give theprescriber knowledge that they will beable to draw upon the next time theyconsider the drug combination.

References

• Glassman PA, Simon B, Belperio P, Lanto A. Improving recognition of drug interactions: benefits and barriers to using automated drug alerts. Med Care. 2002;40(12):1161-1171.
• Ko Y, Malone DC, Skrepnek GH, et al. Prescribers' knowledge of and sources of information for potential drug-drug interactions: a postal survey of US prescribers. Drug Safety. 2008;31(6):525-536.