When Dr. Rosenbaum told her hairdresser that she was studying why some people with heart disease don’t take their medications, he replied, “Medications remind people that they’re sick. Who wants to be sick?” He said his grandmother refuses to take drugs prescribed for her heart condition, but “she’ll take vitamins because she knows that’s what keeps her healthy,” so he tells her that the pills he gives her each night are vitamins.

Other patients resist medications because they view them as “chemicals” or “unnatural.” One man told Dr. Rosenbaum that before his heart attack, he’d switched from the statin his doctor prescribed to fish oil, which unlike statins has not been proved to lower cholesterol and stabilize arterial plaque.

“There’s a societal push to do things naturally,” she said in an interview. “The emphasis on diet and exercise convinces some people that they don’t have to take medications.”

Dr. Bender said, “People often do a test, stopping their medications for a few weeks, and if they don’t feel any different, they stay off them. This is especially common for medications that treat ‘silent’ conditions like heart disease and high blood pressure. Although the consequences of ignoring medication may not show up right away, it can result in serious long-term harm.”

Some patients do a cost-benefit analysis, he said. “Statins are cheap and there’s big data showing a huge payoff, but if people don’t see their arteries as a serious problem, they don’t think it’s worth taking a drug and they won’t stay on it. Or if they hear others talking about side effects, it drives down the decision to take it.”

Cost is another major deterrent. “When the co-pay for a drug hits $50 or more, adherence really drops,” Dr. Bender said. Or when a drug is very expensive, like the biologics used to treat rheumatoid arthritis that cost $4,000 a month, patients are less likely to take them or they take less than the prescribed dosage, which renders them less effective.

Dr. William Shrank, chief medical officer at the University of Pittsburgh Health Plan, said that when Aetna offered free medications to patients who survived a heart attack, adherence improved by 6 percent and there were 11 percent fewer heart attacks and strokes, compared with patients who paid for their medications and had an adherence rate of slightly better than 50 percent.