Many patients who survive myocardial infarction don’t consistently take medications prescribed to prevent a second MI, a large U.S. study suggests.

Researchers asked 7,425 patients how often they took all the drugs physicians prescribed shortly after their heart attack, and about 2,150 of them confessed to following doctors’ orders only some of the time.

While each patient may have unique motivations for deciding whether or not to take medications, it’s not uncommon for people who skip doses or fail to fill prescriptions to do so because they misunderstand the role of the medications in preventing another heart attack, fear side effects, or lack funds to pay for drugs, lead author Dr. Robin Mathews told Reuters Health in an email.

“Ultimately, regardless of the reason, patient non-adherence to medications after a heart attack has been associated with poor outcomes – these can include repeat hospitalization, progression of their underlying disease, or even reduced survival,” said Mathews, a researcher at Duke University Medical Center in Durham, North Carolina.

Mathews and colleagues reviewed drug compliance for MI patients treated at 216 U.S. hospitals between April 2010 and May 2012. During their hospital stay, all of the patients had percutaneous coronary intervention (PCI) and were prescribed antiplatelet therapy.

Six weeks after their MI, 71% of study participants said they consistently took their medications as prescribed. But 25% said they did so only occasionally, and another 4% said they used medications rarely.

Among the people least likely to take their medicine, most cited forgetfulness and about 20% said they didn’t tell their doctor when they stopped taking drugs due to side effects.

While the patients who were least compliant with medication were more likely to die or be hospitalized again within two months of their heart attack, the difference wasn’t large enough to rule out the possibility that this was due to chance.

Because researchers didn’t use data from patients who died within six weeks of their MI, it’s possible that early medication non-compliance was underreported, the authors acknowledge June 2 in the journal Circulation: Cardiovascular Quality and Outcomes.

In addition, they only checked medication adherence at one point in time, and they relied on patients to truthfully report how often they took prescribed drugs.

“There’s a lot going on in the days following a heart attack and often times the discussions about why each medication is important gets diluted by all of the other issues occurring in the hospital at the same time,” said Steven Baroletti, director of pharmacy at MetroWest Medical Center in Framingham, Massachusetts.

Limited insurance coverage or high co-payments, particularly for brand-name drugs, may also keep some cost-conscious patients from filling prescriptions, Baroletti, who wasn’t involved in the study, told Reuters Health by email.

Psychological factors might dissuade other patients, particularly if they are depressed or suffer symptoms of post-traumatic stress disorder from the heart attack, said Dr. Ian Kronish, a researcher at the Center for Behavioral Cardiovascular Health at Columbia University Medical Center in New York who wasn't involved in the study.

“For these patients, the very medicines that can best reduce their chances of another heart attack may serve as unwanted reminders of the heart attack, and may lead some patients to skip their medications,” Kronish said in an email.

Certainly, no two patients are the same in how they decide whether or not to take all the pills doctors prescribe, said Dr. Marie Brown, a researcher at Rush University Medical Center in Chicago who wasn’t involved in the study.

“A typical non-adherent personality does not exist,” she said by email. “The take home message for patients is to be sure to ask why they need the medicine and what will happen if they don’t take it.”