The Supreme Court ruled today that the federal government has the power to prosecute the use of marijuana for medical purposes even in states that have enacted laws permitting it.

In a 6-3 decision, the court agreed with the Bush administration that the regulation of controlled substances, including marijuana, is the province of Congress without exception.

Nonetheless, the ruling does not strike down laws in California and 10 other states allowing medicinal use of marijuana. The court was not asked to declare such statutes illegal.

It means, however, that such laws will not protect anyone from federal prosecution should a U.S. attorney or the Department of Justice bring charges or order raids to stop the practice.

Justice Sandra Day O'Connor, who dissented today, saw the decision as effectively "extinguishing" experiments with medical marijuana laws.

Supporters of medicinal marijuana laws, on the other hand, said they believed that most individuals being treated with marijuana under state laws would be untouched by the ruling.

Daniel Abrahamson, of the Drug Policy Alliance, said that "it will take time for the dust to settle" but that when it does, things will be roughly the same, "an uneasy status quo. . . . States will be able to pass protections for marijuana patients and the federal government will remain free to go after them or not . . . "

Supporters argued that the Controlled Substances Act, under which the federal government prosecutes drug violations, did not specifically bar limited drug use under a physician's supervision when sanctioned by state law. Under these circumstances, they said, the federal government's power to regulate drugs under the Constitution's Commerce Clause is limited.

The argument by the 11 states relied heavily on two Supreme Court cases within the past 10 years, in which the court limited Congress's power to make laws in the name of regulating interstate commerce. The court ruled in 1995 that Congress could not criminalize the possession of guns near schools; in 2000, the court said Congress lacked the authority to give rape victims the right to sue their attackers in federal court. The court said the link between school gun violence or rape -- both of which are already illegal under state law -- and the national economy was too attenuated.

The Bush administration said that whether or not Congress specifically prohibited medicinal drug use, it has broad power over all aspects of drugs and medicine under the Commerce Clause.

The court agreed with the administration in an opinion written by Justice John Paul Stevens, who said that the Controlled Substances Act of 1970 was a valid exercise of federal power by the Congress "even as applied to the troubling facts of this case."

Stevens said that Congress' failure to specifically preclude the practice of medicinal marijuana did not make it exempt from federal regulation.

"We have no difficulty concluding that Congress had a rational basis for believing that failure to regulate the intrastate manufacture and possession of marijuana would leave a gaping hole" in the Controlled Substances Act.

Federal law, Stevens wrote, "designates marijuana as contraband for any purpose . . . The mere fact that marijuana -- like virtually every other controlled substance regulated by the Controlled Substances Act -- is used for medicinal purposes cannot possibly serve to distinguish it from the core activities regulated by the CSA."

The patients in today's case were Angel McClary Raich and Diane Monson, Californians who use marijuana as a medical treatment. Raich has been diagnosed with a number of disorders, including cancer, and used marijuana as a form of relief every other hour while she was awake.

Monson has been using marijuana for relief of severe chronic back pain and muscle spasms caused by a degenerative disease of the spine.

On Aug. 15, 2002, deputies from the Butte County Sheriff's Department and agents from the U.S. Drug Enforcement Agency came to Monson's home and got into a disagreement over the marijuana, with the deputies arguing that it was being legally used under California's Compassionate Use Act while the DEA agents demanded that the drug be destroyed, which they ultimately did.

Monson and Raich, who feared prosecution, sued the U.S. government to stop further raids.

The ruling today reversed a decision of the 9th Circuit Court of Appeals.

Stevens said that the appellate court reached a different conclusion only "by isolating a separate and distinct class of activities that it held to be beyond the reach of federal power, defined as the intrastate, noncommercial cultivation, possession and use of marijuana for personal medical purposes on the advice of a physician and in accordance with state law."

In addition, he said, "limiting the activity to marijuana possession and cultivation in accordance with state law cannot serve to place" the activities "beyond congressional reach." The Constitution's Supremacy Clause, he said, "unambiguously provides that if there is any conflict between federal and state law, federal law shall prevail."

O'Connor was joined in dissent by Chief Justice William H. Rehnquist and Justice Clarence Thomas.

"This case exemplifies the role of states as laboratories," she wrote.

"The states' core police powers have always included authority to define criminal law and to protect the health, safety, and welfare of their citizens. Exercising those powers," O'Connor said in her opinion. "California has come to its own conclusion about the difficult and sensitive question of whether marijuana should be available to relieve severe pain and suffering. Today the court sanctions an application of the federal Controlled Substances Act that extinguishes that experiment, without any proof" that it "has a substantial effect on interstate commerce and is therefore an appropriate subject of federal regulation."

O'Connor said she would have opposed California's medical marijuana law if she were a voter or a legislator. But she said the court was overreaching to endorse "making it a federal crime to grow small amounts of marijuana in one's own home for one's own medicinal use."