WOONSOCKET, R.I. — Of patients prescribed a new medication by their doctor, 24% did not fill the prescription, a study by CVS Caremark Corp., Harvard University, and Brigham and Women’s Hospital researchers found.

According to CVS Caremark, most prior research of patients who do not take medications as prescribed examined patient behavior after the filling of a first prescription. However, electronic prescribing has enabled the tracking of initial prescriptions that previously may have been undetected.

The new CVS Caremark-sponsored study evaluated over 423,000 e-prescriptions written in 2008 by 3,634 doctors for more than 280,000 patients from all 50 states. The study team matched e-prescriptions with resulting claims data or, in the case of those not filling the prescription, used the lack of a claim within six months to identify primary nonadherence.

“CVS Caremark is looking at this issue from every angle,” says study coauthor Troyen Brennan, executive vice president and chief medical officer at CVS Caremark. “This study reviewed factors that might cause patients to ignore that first fill so we can arm health care providers with information to proactively address the problem.”

Researchers found that key factors behind primary nonadherence included out-of-pocket cost, as patients who received prescriptions for medications not on their health plan formulary were more likely not to fill their first prescription.

Also, prescriptions sent directly to pharmacies or mail-order systems were more likely to be filled than e-scripts that doctors printed and gave to patients, and patients living in higher-income areas were more likely to fill prescriptions for new medications.