Most doctors don't give enough painkillers to ease the suffering of cancer victims, and nearly two-thirds admit they do a poor job of even learning if their patients hurt, said a study.

The major reason doctors don't treat pain aggressively enough is their fear that they will not be able to deal with side effects of the medicines, the study, released this week, found.A variety of other factors also play a role, including poor training in pain control and failure to pay attention to patients' misery.

"Pain has been a low priority. That's one of the problems," said Dr. Jamie H. Von Roenn of Northwestern University, who directed the study.

The study was based on a 1989 survey of 1,800 cancer specialists who belong to the Eastern Cooperative Oncology Group, a major cancer study organization. Mailed questionnaires were returned by 1,177 physicians who had treated more than 70,000 cancer patients during the previous six months.

"The most common fear of cancer patients is pain. They are often under-treated. Dr. Von Roenn's paper gives us some of the reasons for this," said Dr. Thomas H. Cartwright of Florida's Ocala Oncology Center.

Dr. Gary R. Shapiro, an ethicist at Abbott Northwestern Hospital in Minneapolis, said he believes patients' fear of cancer pain - along with doctors' inability to control it - has contributed to some highly publicized cases of physicians helping patients commit suicide.

"If we had better education and control of pain, much of the perceived need for active euthanasia would vanish," Shapiro said.

Von Roenn estimated that if used competently, pain medicines can relieve the agony of 80 percent to 90 percent of cancer patients.

Among the findings of her report, presented at the annual meeting of the American Society of Clinical Oncology:

-About 60 percent of doctors acknowledged that poor pain assessment is a major barrier to controlling pain. Von Roenn said doctors could make questions about pain a regular part of their daily rounds but rarely do so.

-Eighty-five percent of doctors surveyed said they believe that the majority of cancer patients in the United States are under-medicated for pain.

-Nearly half said that patients' reluctance to take pain pills contributes to the problem.

Von Roenn said patients sometimes confuse needed pain medication with illegal drugs.

-About one-third of doctors said they do not prescribe the highest levels of pain medicine unless they think their patients have less than six months to live.

-Two-thirds cited concerns over side effects - largely nausea and confusion - as their reason for limiting use of analgesics. Fear of addiction, once a major concern, is no longer cited as a significant worry.

Dr. Harmon J. Eyre of the University of Utah said some patients delay having cancer diagnosed because they fear the pain of treatment. However, he said doctors are becoming more concerned about the problem.