Drug interactions avoided-a useful indicator of good prescribing practice

To develop an index of quality prescribing in general practice by investigating the incidence of potential drug interactions when medicines were coprescribed within the State supported General Medical Services (GMS) in Ireland.

We determined an odds ratio (OR), as a measure of the relative risk of being exposed to a potential interaction, comparing the use of the H2-receptor antagonist, cimetidine, with that of the noninteracting agents ranitidine, famotidine and nizatidine in users and nonusers of warfarin, phenytoin and theophylline.

In 86 510 prescriptions for the H2-receptor antagonists potentially interacting drugs were dispensed to 8188 (9%) patients in the Eastern Health Board Region of the GMS. We found that prescribers were significantly less likely to use cimetidine (OR = 0.20,95% CI 0. 17-0.21, P < 0.001) in those patients who were coprescribed warfarin, suggesting good prescribing practice within the GMS. Similarly there was preferential use of the noninteracting H2-receptor antagonists in patients receiving phenytoin or theophylline and the extent of this selective prescribing was in keeping with the rank order of severity of interaction with these drugs. This novel pharmacological index may be a sensitive marker of good prescribing practice.