Douglas Jenson, 75, of Canby, Ore., has taken Gleevec for 10 years for leukemia. He goes for a blood test once every three months and sees his oncologist every six months, but is healthy enough to go whitewater rafting.

Making it even easier, Mr. Jenson gets his Gleevec free because he participated in an early clinical trial of the drug. Otherwise it would cost more than $40,000 a year.

While Mr. Jenson has been diligent about taking his five capsules every day at lunchtime, research indicates that many patients on the oral drugs do not consistently take the proper dose. One study, for example, found that Gleevec patients, on average, were taking only 75 percent of their prescribed doses.

Some cancer patients skip pills or stop taking them completely  whether because of costs, forgetfulness, side effects, complicated regimens or other factors.

“When I first started looking into this, I thought, ‘People with cancer have too much to lose, how can they not take their drugs?’ ” said Dr. Ann Partridge, an oncologist at Dana-Farber Cancer Institute in Boston.

Some other cancer patients, meanwhile, end up taking too many pills.

Gayne Ek of Allen, Tex., said he once skipped all of his Gleevec capsules for six weeks. Then, with the stockpile of capsules he accumulated, he took twice the prescribed dose for six weeks, hoping it would be more effective. It was not.

For many patients, though, the main challenge is not taking their pills, but paying for them. Under Medicare, most oral cancer drugs are covered by the Part D prescription drug program, which has a 25 percent co-payment. It also has the annual “doughnut hole”  reached when a patient’s total drug costs hit $2,700, after which the patient must shoulder the next $3,000 or so before coverage resumes.