Examining data on drug prescriptions to more than 5 million Americans over six months, the researchers found that just over 3 percent were filled but not picked up from the pharmacy.
Although that overall figure is low, the results suggest that such prescription "abandonment" contributes to the larger problem of patients' non-adherence to their medication regimens.
"I'd say it's a small but meaningful contribution," said lead researcher Dr. William H. Shrank, of Harvard Medical School and Brigham and Women's Hospital in Boston. That's partly because certain patients had increased odds of not picking up their prescriptions, Shrank noted in an interview.
He and his colleagues found that those prescribed drugs with higher price tags were more likely to leave the medications unclaimed at the pharmacy, as were those receiving first-time prescriptions for a particular type of medication.
And that, Shrank said, points to areas where there is room for improvement. A number of studies have shown that people with chronic health problems, including high blood pressure, diabetes and heart disease, commonly fail to take their medications as prescribed.
To find out why, Shrank's team wanted to look at one aspect of the larger picture: how often patients simply never pick up their prescriptions from the pharmacy. Their study, published in the Annals of Internal Medicine, also looked for clues that might explain the patients' behavior and flag problems that doctors and pharmacists could address.
The researchers used data from CVS Caremark, a national retail pharmacy chain and pharmacy benefits manager that also funded the study. They focused on 10.3 million prescriptions filled by more than 5 million patients in 2008.
Of those prescriptions, 3.3 percent were abandoned -- meaning the medication was bottled but returned to stock when the patient did not pick it up after two weeks.
Not all those patients necessarily went untreated. In just under half of cases, the patient bought a prescription for another medication in the same drug class within 30 days, sometimes at a different pharmacy.
But 1.8 percent of all prescriptions were returned to stock and apparently not replaced with an alternative.
Certain prescriptions had a higher likelihood of not being picked up: New prescriptions were nearly three times as likely as others to go unclaimed, for instance. Patients with complex prescription regimens for multiple health problems were more likely to abandon one or more of their medications.
And prescriptions for opiate drugs were least likely to be abandoned -- perhaps because patients' need for painkillers is more acute, the authors speculate, or the drugs' potential for resale is greater.
The strongest predictor of prescription abandonment, however, was price: Drugs that cost more than $50, and those with a co-payment of $40 to $50, were three to five times more likely to go unclaimed than medications with no patient co-payment.
It's not surprising that patients were more likely to abandon higher cost drugs, according to Shrank. "I think this is telling us that 'sticker shock' is real," he said.
"Patients often don't know the cost of a drug until they arrive at the pharmacy." To help avoid that, Shrank suggested that when patients are given a new prescription, they ask the doctor about the price.
A lower cost alternative might be available. Pharmacists could also work more closely with doctors to make them aware of drug prices and lower cost alternatives, the authors write.
As for the greater likelihood of new prescriptions being abandoned, that could signal that some patients do not fully understand why they are being put on a new drug, according to Shrank.
He again advised people to talk to their doctors, and ask questions any time they do not understand the reasons for a drug prescription.
Shrank and his colleagues also found that prescriptions sent to the pharmacy electronically had a 64 percent greater chance of abandonment than those prescribed the old-fashioned way -- either hand-delivered by patients or phoned or faxed in by the doctor.
But, Shrank said, "that does not mean that electronic prescriptions are bad."
One reason for their higher likelihood of abandonment is that, unlike paper prescriptions brought in by patients, electronic prescriptions arrive at the pharmacy regardless of a patient's intention to ultimately pick up the drug.
But it's also possible that patients not given a paper prescription are more apt to forget about the medication.
Shrank said that it might be helpful for doctors who use electronic prescribing to give patients written reminders to make sure they remember that their medication will be awaiting them at the pharmacy.
Overall, the study findings "shed light on an important component of medication non-adherence," according to an editorial published with the report.
One remaining question is how such prescription abandonment ultimately affects people's health, write Drs. Michael D. Murray, of Purdue University College of Pharmacy in Indianapolis, and Jeff Harrison of the University of Auckland in New Zealand.
In some cases, they note, a patient's decision not to pick up a drug -- out of concern for side effects, for instance, or because he or she already has an adequate supply -- may turn out to have positive effects, or none at all.
On the flip side, Murray and Harrison point out, the fact that a patient picks up a prescription does not necessarily mean that he or she ends up taking it.
Shrank said that studies need to keep investigating the various and complex reasons that people do not use their medications as prescribed.
"We need to have a better understanding of how we can help patients adhere to their medications," he said.
SOURCE: Annals of Internal Medicine, online November 15, 2010.