Co-prescribing of contraindicated and use-with-caution drugs in a national cohort of new users of simvastatin: how well are prescribing guidelines followed?

AIM: To describe the use of contraindicated and use-with-caution medicines among new users of simvastatin.

METHODS: We used information from Ministry of Health national datasets to establish a cohort of all patients aged >18 years who initiated simvastatin use between January 2006 and December 2013 (n=349,371). We estimated the cumulative incidences of the first dispensing of contraindicated and use-with-caution medicines during simvastatin use, and explored factors associated with co-prescription, using Kaplan-Meir and Cox regression methods, respectively.

RESULTS: Eleven percent and 16% of patients were dispensed a contraindicated and use-with-caution medicine, respectively, during the first two years of simvastatin use; by seven years, the figures were 17% and 26%. Thirty-six percent of patients were co-prescribed a contraindicated medicine on >1 occasion; the corresponding proportion for use-with-caution medicines was 84%. For a substantial proportion of those co-prescribed a use-with-caution medicine, the concomitant daily dose of simvastatin exceeded the maximum dose recommended at the time of prescribing. In the majority of cases, the prescriber of simvastatin and the contraindicated or use-with-caution medicine were the same. Co-prescribing of contraindicated medicines varied by sex, age, ethnicity and comorbidity.

CONCLUSIONS: The prescription of contraindicated and use-with-caution drugs to patients taking simvastatin is not uncommon in New Zealand.