In the first New Zealand study of its kind, it was found that, after seven years, 17% of patients on the statin, simvastatin, had also been prescribed contraindicated drugs that could increase the risk of side effects.
One of the study’s authors, Dr Lianne Parkin, of the Otago preventive and social medicine department, said she hoped the study would raise awareness of the matter.
The situation was similar overseas, and there was "room for improvement", Dr Parkin said.
Simvastatin is a cholesterol-lowering drug, widely used in New Zealand to prevent heart attacks and stroke.
But some other medicines can inhibit the activity of the enzyme (CYP3A4) that metabolises simvastatin.
This can lead to high levels of simvastatin in the blood, which in turn increases the risk of side effects. A key question in prescribing other drugs for patients on simvastatin was "Is this the best drug?" or were there similar alternatives which did not conflict with the guidelines, did not inhibit CYP3A4 enzyme breakdown, and which "might be a better choice", she said.
Prescribing guidelines state that some such inhibiting medicines — termed "contraindicated medicines" — should not be prescribed to people taking simvastatin.
Contraindicated medicines included some antibiotics and antifungal medicines.
Also, some "use-with-caution medicines" should only be used with careful management of the simvastatin dose.
These included some drugs used to manage cardiovascular conditions, including high blood pressure, abnormal heart rhythms and to reduce triglycerides, a type of blood lipid.
In the nationwide study, just published in the New Zealand Medical Journal, researchers from the Pharmacoepidemiology Research Network found that despite prescribing guidelines and patient management software, which alerts prescribers to potential drug interactions, 11% of patients were dispensed a contraindicated medicine during the first two years of simvastatin use and 16% received a use-with-caution medicine. After seven years of statin use, 17% of patients had been dispensed a contraindicated medicine and 26% had received a use-with-caution medicine.
The study was based on anonymised national pharmaceutical dispensing and health data for 349,371 people who started taking simvastatin between January 2006 and December 2013.
Co-investigator Joshua Quon is a student at the Dunedin School of Medicine.