More important, it explains why so many patients don’t get better, suffer surprising relapses or even die when they are given drug prescriptions that should keep their disorders under control.

Studies have shown that a third of kidney transplant patients don’t take their anti-rejection medications, 41 percent of heart attack patients don’t take their blood pressure medications, and half of children with asthma either don’t use their inhalers at all or use them inconsistently.

“When people don’t take the medications prescribed for them, emergency department visits and hospitalizations increase and more people die,” said Bruce Bender, co-director of the Center for Health Promotion at National Jewish Health in Denver. “Nonadherence is a huge problem, and there’s no one solution because there are many different reasons why it happens.”

For example, he said parents often stop their children’s asthma treatment “because they just don’t like the idea of keeping kids on medication indefinitely.” Although a child with asthma may have no apparent symptoms, there is underlying inflammation in the lungs and without treatment, “if the child gets a cold, it can result in six weeks of illness,” Dr. Bender explained.

When Dr. Lisa Rosenbaum, a cardiologist at Brigham and Women’s Hospital in Boston, asked patients who had suffered a heart attack why they were not taking their medications, she got responses like “I’m old-fashioned — I don’t take medicine for nothing” from a man with failing kidneys, peripheral vascular disease, diabetes and a large clot in the pumping chamber of his heart. Another common response: “I’m not a pill person.”

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.

“There are so many reasons patients don’t adhere — the prescription may be too complicated, they get confused, they don’t have symptoms, they don’t like the side effects, they can’t pay for the drug, or they believe it’s a sign of weakness to need medication,” Dr. Shrank said. “This is why it’s so hard to fix the problem — any measure we try only addresses one factor.”

Still, there is hope for improvement, he said. Multiple drugs for a condition could be combined into one pill or packaged together, or dosing can be simplified. Doctors and pharmacists can use digital technology to interact with patients and periodically reinforce the importance of staying on their medication.With fear of side effects a common deterrent to adherence, doctors should inform patients about likely side effects when issuing a prescription. Failing that, patients should ask: “What, if any, side effects am I most likely to encounter?

Forgetting to take a prescribed drug is a common problem, especially for those ambivalent about taking medication. Patients can use various devices, including smartphones, to remind them to take the next dose, or use a buddy system to make adherence a team sport. Dr. Shrank suggested making pill-taking a habit, perhaps by putting their medication right next to their toothbrush