While antibiotic prescriptions were often filled, drugs prescribed for headache and coronary artery disease were filled about half the time, researchers found.

A new Canadian study suggests nearly one in three new prescriptions goes unfilled, with expensive drugs and medications used to control some chronic conditions more likely not to be taken as directed.

The first author of the paper admits that she and her colleagues were startled at the magnitude of the gap between prescriptions written and prescriptions filled.

Robyn Tamblyn says doctors need to keep this fact in mind because they may assume a patient's drugs aren't working when in fact the patient isn't taking the prescribed medication.

She says those patients may be too embarrassed or too intimidated to share that information with their doctors.

Tamblyn is an epidemiologist who teaches in McGill University's school of medicine; her paper was published Monday in the journal Annals of Internal Medicine.

She and her colleagues found that antibiotic prescriptions are often filled. When the data were analyzed on a
condition-by-condition basis, prescriptions for medications for urinary tract infections were least likely to go unfilled.

But drugs prescribed for headache and coronary artery disease were only filled about half the time and prescriptions for
antidepressants were filled only 37 per cent of the time.

The Quebec study didn't look at why people didn't fill their prescriptions; Tamblyn says she and her colleagues plan to do a followup study to ask patients that question.

But there are hints in the data. Prescriptions for higher-priced drugs were more likely to go unfilled. And patients who had higher drug co-payments were more likely not to fill prescriptions.

Older patients were more likely to follow doctor's orders and those who were taking multiple medications were also more likely to fill prescriptions.

Dr. David Juurlink, a specialist in internal medicine and clinical pharmacology, says while the one-in-three number looks
surprising at first, the reality is doctors write too many prescriptions.

Juurlink, who practises at Sunnybrook Health Sciences Centre in Toronto, says some prescriptions are written with clear instructions that patients should only fill them if necessary. So in some cases not filling a prescription is the rational approach, he says, though he acknowledges a portion of these prescriptions should have been filled and were not.

The study was funded by the Canadian Institutes of Health Research.