PORTLAND — A new Oregon Health & Science University study suggests that one out of four terminally ill patients who request a prescription for a lethal overdose under Oregon’s landmark assisted suicide law may suffer from depression.

The study, published Tuesday in the online issue of the British Medical Journal, indicates stronger screening may be needed for the terminally ill who ask a doctor for assistance under the law, researchers said.

Supporters of the law, however, said the Oregon Death With Dignity Act already requires rigorous screening and questioned the methodology of the study.

“If anything, the study confirms the law is well written and that it works well,” said Barbara Coombs Lee, co-author of the act and president of Compassion & Choices.

The study was released as voters in Washington state consider an initiative on the November ballot that would make Washington the second state in the nation to allow assisted suicide.

Oregon’s law has twice been approved by its voters and upheld by the U.S. Supreme Court in 2006, despite a challenge from the Bush administration.

Last week, California Gov. Arnold Schwarzenegger signed a law requiring doctors to offer terminally ill patients information about their end-of-life options, including the right to cut off treatment.

Lee, a former nurse who is now an attorney, said the OHSU study included questions about the typical symptoms of depression in a healthy person, such as weight loss and fatigue. But many terminally ill patients are suffering those symptoms as a result of a disease, such as cancer.

“The main question under the Death With Dignity Act is not whether a patient is eating well or has a sunny outlook, but whether they have the sound judgment to make an informed choice,” Lee said.

Dr. Linda Ganzini, the study’s lead author, agreed that some level of depression is expected from a patient with a fatal illness.

“In fact, the finding that three-quarters of requesting patients have no depression is somewhat surprising,” Ganzini said.

But she said the study “supports that more rigorous screening for depression should occur as part of the physician-assisted suicide process.”

Tim Rosales, spokesman for Californians Against Assisted Suicide, said the study supports critics who say the Oregon law relies too much on self-reporting by doctors and patients without enough oversight.

“For those folks who were depressed, there’s no telling whether had they gotten treatment for that depression, they would have still made that choice,” Rosales said.

Eli Stutsman, a Portland lawyer and co-author of the Oregon act who helped win the Supreme Court ruling, said the law recognizes the difference between depression and impaired judgment.

“Simply put, if the physician suspects depression, then the referral for counseling becomes mandatory,” Stutsman said. “If the patient’s judgment is impaired, the process stops. The patient will not qualify.”

For the study, psychiatrists at OHSU evaluated 58 Oregonians who had requested assistance from their doctor or an organization. Most of the patients had been diagnosed with terminal cancer or amyotrophic lateral sclerosis, often referred to as Lou Gehrig’s disease

The patients were evaluated through two forms of standardized depression and anxiety rating scales.

The screenings determined that 26 percent, or 14 patients, suffered from depression and 22 percent, or 13 patients, suffered from anxiety.

By the end of the study, 42 patients had died, most of them due to disease. Only 18 patients received prescriptions for lethal medication, and nine used it.