WASHINGTON – The Supreme Court questioned whether state medical marijuana laws might be abused by people who aren’t really sick as it debated on Monday whether the federal government can prosecute patients who smoke pot on doctors’ orders.

The stakes are high on both the government level – 11 states have passed medical marijuana laws since 1996 – and the personal.

In the courtroom watching the argument was Angel Raich, an Oakland, Calif., mother of two who said she tried dozens of prescription medicines to ease the pain of a brain tumor and other illnesses before she turned to marijuana. She and another ill woman, Diane Monson, filed a lawsuit to protect their access to the drug after federal agents confiscated marijuana plants from Monson’s yard.

Their attorney, Randy Barnett of Boston, told the justices that his clients are law-abiding citizens who need marijuana to survive. Marijuana may have some negative side effects, he said, but seriously sick people are willing to take the chance because the drug helps them more than traditional medicines.

The justices refused three years ago to protect distributors of medical marijuana from federal anti-drug charges. They are confronting a more personal issue this time – the power of federal agents to go after sick people who use homegrown cannabis with their doctors’ permission and their states’ approval.

A defeat for the two California women might undermine laws passed by California and 10 other states and discourage other states from approving their own.

A loss for the government, on the other hand, could jeopardize federal oversight of illegal drugs and raise questions in other areas such as product safety and environmental activities. A Bush administration lawyer told the justices they would be encouraging people to use potentially harmful marijuana if they were to side with the women.

“If they’re right, then I think their analysis would extend to recreational use of marijuana, as well as medical use of marijuana, and would extend to every state in the nation, not just those states that made it lawful,” said Paul Clement, acting solicitor general.

Justice David H. Souter said an estimated 10 percent of people in America use illegal drugs, and states with medical marijuana laws might not be able to stop recreational users from taking advantage.

Justice Stephen Breyer said the government makes a strong argument that as many as 100,000 sick people use marijuana in California, and “when we see medical marijuana in California, we won’t know what it is. Everybody’ll say, ‘Mine is medical.’ Certificates will circulate on the black market. We face a mess.”

And Justice Antonin Scalia said there are many people with “alleged medical needs.”

Despite the tenor of the debate, the case is hard to predict. The justices will rule before next summer.

The marijuana users won in the San Francisco-based 9th U.S. Circuit Court of Appeals, which ruled that federal prosecution of medical marijuana users is unconstitutional if the pot is not sold, transported across state lines or used for nonmedicinal purposes.

Justice Ruth Bader Ginsburg said the federal government has a stake in interstate commerce, but with the California medical marijuana patients: “Nobody’s buying anything. Nobody’s selling anything.”

Her colleague, Justice Sandra Day O’Connor, observed that homegrown medical marijuana never makes it to the interstate market.

Conservatives like Chief Justice William H. Rehnquist, Justice Clarence Thomas and Scalia generally have supported states’ rights to set their own policies.

Rehnquist, who is undergoing treatment for thyroid cancer, missed Monday’s argument and is not expected to return to the court until January, at the earliest.