WASHINGTON - Attorney General John Ashcroft gave federal drug agents the go-ahead yesterday to take action against doctors who help terminally ill patients die, a move aimed at undercutting Oregon's unique assisted-suicide law.

The decision, outlined in a letter to Drug Enforcement Administration chief Asa Hutchinson, would allow the revocation of drug licenses of doctors who participate in an assisted suicide using a federally controlled substance.

Ashcroft's letter reverses a June 1998 order by his predecessor, Janet Reno, who barred agents from moving against doctors who used Oregon's law.

Ashcroft said assisted suicide is not a "legitimate medical purpose" for prescribing, dispensing or administering federally controlled substances. However, he said pain management is a valid medical use of controlled substances.

Ashcroft based his decision on a unanimous Supreme Court ruling in May that said there is no exception in federal drug laws for the medical use of marijuana to ease pain from cancer, AIDS and other illnesses.

The court didn't change state laws allowing patients to use marijuana for medical reasons but made the drug harder to obtain by denying patients the right to claim "medical necessity" as a reason to circumvent a 1970 law regulating controlled substances.

Under Oregon's Death With Dignity Act, doctors may provide - but not administer - a lethal prescription to terminally ill adult state residents. It requires that two doctors agree the patient has less than six months to live, has voluntarily chosen to die and is able to make health care decisions.

At least 70 terminally ill people have ended their lives since the law took effect in 1997, according to the Oregon Health Division. All have done so with a federally controlled substance such as a barbiturate.

Advocates for the Oregon law worry Ashcroft's decision will limit medical professionals' ability to treat pain because they will be less likely to prescribe large amounts of a medication to those who might need it.

"The federal government has way overstepped its boundaries and has undermined good health care practices across the country, not just in Oregon," said Scott Swenson, executive director of Oregon Death with Dignity. "What they have done is to federalize medicine."

In a 1998 letter to Rep. Henry Hyde, R-Ill., Reno said she found no evidence the Controlled Substances Act was intended to displace states as the primary regulators of the medical profession or override a state's authority to determine what constitutes a legitimate medical practice.

Since then, conservative, religious and anti-abortion groups have mounted a campaign to try to block the Oregon law. Sen. Don Nickles, R-Okla., pushed a bill last session that would have done what Ashcroft ordered. The measure, stridently opposed by Sen. Ron Wyden, D-Ore., never reached the floor for a vote.

Oregon voters twice approved physician-assisted suicide in referendums during the 1990s. The Supreme Court in June 1997 upheld bans on assisted suicide in New York and Washington state, but left it up to states to decide whether to allow the practice.