The closely watched case was an appeal by the Bush administration in a case involving two seriously ill California women who use marijuana. The court said the prosecution of marijuana users under the federal Controlled Substances Act was constitutional.
''I'm going to have to be prepared to be arrested," said Diane Monson, one of the women involved in the case.
Stevens said the court was not passing judgment on the potential medical benefits of marijuana, and he noted ''the troubling facts" in the case. Monson's backyard crop of six marijuana plants was seized by federal agents in 2002, although the California law was on Monson's side.
In a dissent, Justice Sandra Day O'Connor said that states should be allowed to set their own rules.
Justice Department spokesman John Nowacki said the department is pleased with the court's decision, but would not comment on whether federal prosecutors would pursue cases against people like Monson.
Under the Constitution, Congress may pass laws regulating a state's economic activity so long as it involves ''interstate commerce" that crosses state borders. The California marijuana in question was homegrown, distributed free to patients, and did not cross state lines.
''Our national medical system relies on proven scientific research, not popular opinion. To date, science and research have not determined that smoking marijuana is safe or effective," John Walters, director of National Drug Control Policy, said yesterday.
Stevens said there are other legal options for patients, ''but perhaps even more important than these legal avenues is the democratic process, in which the voices of voters allied with these [California women] may one day be heard in the halls of Congress."
California's medical marijuana law, passed by voters in 1996, allows people to grow, smoke, or obtain marijuana for medical needs with a doctor's recommendation. Alaska, Colorado, Hawaii, Maine, Montana, Nevada, Oregon, Vermont, and Washington state have similar laws.
In those states, doctors generally can give written or oral recommendations on marijuana to patients with cancer, HIV, and other serious illnesses.
