WASHINGTON – Supreme Court justices cast doubt yesterday on the power of state legislatures to defy federal drug laws by allowing patients to use marijuana to relieve a variety of medical ailments.

The federal government has asked the justices to overturn a California court order that prevents the prosecution of two women who smoke the drug on doctors’ orders under the state’s 1996 Compassionate Use Act.

Similar laws in 10 states have created “islands” of marijuana use that interfere with Congress’s ability to regulate the nationwide drug trade, argued the acting federal solicitor general, Paul Clement.

A lawyer for the patients, Randy Barnett, countered that the Rehnquist court’s own opinions say that Congress cannot abuse its constitutional power to regulate interstate commerce by reaching noneconomic activities that do not cross state lines. The private cultivation and consumption of marijuana is such an activity, he said.

Several justices suggested yesterday that the federal government does have the power to enforce the federal ban on drug use, despite state laws to the contrary, because the private possession of cannabis affects demand for marijuana in the national market, estimated to be worth $10.5 billion. Private marijuana possession could also undermine other national drug laws, they said.

“Since it is grown in the home, it doesn’t have to be bought elsewhere,” Justice Scalia observed.

While Mr. Barnett said only a small number of patients with severe illnesses are prescribed marijuana to relieve their pain, Justice Souter was one of several justices who appeared to agree with the federal government’s argument that many people could be affected. If 100,000 Californians undergo chemotherapy at a given time, the judge said, it is “not implausible to think” that many of them could turn to the drug for relief from nausea.

Justice Kennedy agreed with the government that it is a “sensible assumption” that many patients would purchase the drug “on the street,” even though the law is intended to allow only private cultivation and consumption.

Justice Breyer suggested that the medical marijuana movement should focus its efforts on persuading the Federal Drug Administration to approve marijuana for medical uses.

“Medicine by regulation is better than medicine by referendum,” he said.

Mr. Barnett responded that the federal government had stymied medical research of marijuana by limiting access to the drug. He made the case that the marijuana plants seized from his clients had never crossed state lines, but Justice Scalia pointed out that federal law may ban the mere possession of some objects, as is the case for products such as ivory and eagle feathers taken from endangered species.

Justice O’Connor, who has taken a narrow view of the congressional power to regulate under the commerce clause, took exception to the government’s arguments.

“We have to assume that the state of California will enforce its laws,” she said, when Mr. Clement argued that some of the privately grown marijuana “would be diverted” into the drug trade, undermining the government’s attempts at drug regulation.

She also disagreed with Mr. Clement’s interpretation of a key 1942 case, Wickard v. Filburn, in which the court ruled that wheat grown on a private farm fell under federal regulation because it affected the broader national market.

Justice O’Connor said the wheat in that case affected the market for the narrow reason that some was sold and fed to cattle destined for sale, but several other justices said the wheat was held to have an economic impact because it otherwise would have been purchased on the open market.

Justice Ginsburg also expressed sympathy for the patients’ plight, noting that court records suggested no other drugs worked on their ailments.

Despite the seriousness of the subject matter, the hearing at time drew nervous laughter from the justices and lawyers alike.

At one point, Justice Kennedy asked each lawyer whether he believed the “street price” of marijuana would go up or down if the patients had their way. Mr. Clement predicted it would go down and the drug would become more accessible to new users, while Mr. Barnett argued that any reduction in price would be “trivial” because the number of patients using it would be very small.

In another instance, Mr. Barnett compared private drug use to another vice.

“Prostitution is economic activity, but marital relations are not economic activity,” he said – adding that this was true even though the supply of the former affects demand for the latter.

The case, known as Ashcroft v. Raich, was sparked by injunctions against Angel McClary Raich, an Oakland woman who uses marijuana every two hours to relieve symptoms associated with an inoperable brain tumor, and Diane Monson, of Oroville, Calif., who has been using marijuana for five years to relieve chronic pain and back spasms.

Speaking to reporters after the hearing, Ms. Monson observed that the justices “are all aging,” and could find themselves “in medical situations” where marijuana could be a benefit.

“I think they ought to be very careful what they choose today,” she said.

Both women were dressed in conservative clothing and emphasized that they do not use marijuana for recreation. Ms. Raich said she was initially offended when a nurse suggested she speak to her doctor about using the drug to deal with her symptoms.

She said she takes the drug to be well enough to spend time with her children, the youngest of whom was 5 years old when she was diagnosed.

“If it wasn’t for cannabis, I wouldn’t be here today fighting for my rights,” she said.

Chief Justice Rehnquist, who is receiving chemotherapy and radiation for thyroid cancer, did not participate in the oral arguments. The second most senior judge, Justice Stevens, presided over the hearing. He said the chief would participate in judging the case by reviewing written briefs and transcripts of the hearing.

A group of demonstrators carried signs saying “Don’t arrest patients,” while others carried anti-drug signs such as, “1,000 drug-related deaths a week, most begun with pot.”

A decision is expected by June.