WASHINGTON – The Supreme Court dealt a blow to the medicinal marijuana movement yesterday, ruling that strict federal drug laws trump state schemes that allow ailing patients to grow and consume cannabis for therapeutic purposes.

The 6-3 ruling gives a green light for federal officials to prosecute marijuana users in 10 states that allow the medicinal use and private cultivation of the substance.

The constitutional power of Congress to regulate commerce between the states allows federal lawmakers to regulate marijuana – even when it is produced and consumed entirely within California’s borders, the majority ruled.

Congress may regulate purely local activities “that are part of an economic class of activities that have a substantial effect on interstate commerce,” Justice Stevens wrote.

Dissenting in the case were three Republican-appointed judges, Chief Justice Rehnquist and Justices O’Connor and Thomas, who said the decision was a setback for the doctrine of states’ rights that had been strengthened in recent years and considered a key legacy of the Rehnquist court.

The court had in the past struck down federal laws banning guns in schools and allowing battered women to sue in state courts, ruling that they were unauthorized by the commerce clause. The dissenters said yesterday’s opinion was “irreconcilable” with the court’s own past federalism rulings.

The majority decision invites intrusive federal regulation and “stifles an express choice by some states, concerned for the lives and liberties of their people, to regulate medical marijuana differently,” Justice O’Connor wrote.

California voters had sanctioned medicinal marijuana use by ballot initiative, leading the state Legislature to pass the Compassionate Use Act of 1996.

Noting that the American federalist system of government “promotes innovation” by states wishing to try “novel social and economic experiments without risk to the rest of the country,” Justice O’Connor wrote that “this case exemplifies the role of states as laboratories.”

Judges on each side of the case disclosed that their personal policy preferences contradicted their legal conclusions. Democratic appointee Justice Stevens said he found the facts of the case “troubling” and urged the patients to take their case to lawmakers so their voices “may one day be heard in the halls of Congress.” Meanwhile, Justice O’Connor, a former Republican lawmaker, sided with the patients, despite noting that she was opposed to both the ballot initiative and the law.

Justice Scalia, a supporter of states’ rights, surprised some observers by penning a concurring opinion that sided with the federal power, explaining that the federal government may regulate activities that have a substantial impact on interstate commerce.

“One need not have a degree in economics,” agreed Justice Stevens, “to understand why a nationwide exemption for the vast quantity of marijuana (or other drugs) locally cultivated for personal use (which presumably would include use by friends, neighbors, and family members) may have a substantial impact on the interstate market for this extraordinarily popular substance.”

The attorney general of California, Bill Lockyer, said he was “disappointed” with the decision, which he believes “contradicts the will of California voters and limits the right of states to provide appropriate medical care for its citizens.”

Declaring that California’s law “still stands in our state,” Mr. Lockyer said the decision nonetheless meant that “ill Californians will continue to run the risk of arrest and prosecution under federal law when they grow or use marijuana as medicine.”

One of the patients who brought the case, 39-year-old Angel Raich, of Oakland, who suffers from a brain tumor and other ailments, said she would continue to use the drug, which she claims keeps her alive by stimulating her appetite.

The other patient, Diane Monson, 48, of Oroville, said she would continue to use the drug to relieve her chronic back pain. The Supreme Court did not rule on the question of whether denying her the drug would violate her due process rights, a claim she vowed to pursue in lower courts.

The majority’s decision delighted proponents of stringent drug laws. “This … is a victory for the future of our children and the hope for a drug-free America,” the executive director of the Drug Free America Foundation, Calvina Fay, said.

But proponents of limited federal regulation were disappointed with the decision. “After this opinion, it’s hard to imagine anything Congress cannot regulate,” the director of the center for constitutional studies at the Cato Institute, Roger Pilon, said. The libertarian think tank had filed an amicus brief in the case, and one of its senior fellows was co-counsel for Ms. Raich.

Mr. Pilon praised the dissenting conservatives for not “throwing these women overboard to save the war on drugs,” but accused the more liberal judges of having done so “to save the modern regulatory state.”