The move by the Justice Department ended months of uncertainty over how far the Obama White House planned to go in reversing the Bush administration's stance on the issue, which held that authorities should continue to enforce federal drug laws even in states with medical-marijuana laws on the books.

In new guidelines circulated yesterday, the Justice Department told prosecutors and federal drug agents that they had more important things to do than to arrest people as long as they were obeying the laws of states that allow some use or sale of medical marijuana.

The move clarifies what some critics had said was an ambiguous position of the Obama administration, especially in California, where authorities raided numerous clinics and made arrests over the years. Some of those raids followed Obama's inauguration in January, after, as a presidential candidate, he had pledged to stop them.

The American Civil Liberties Union and other advocacy groups welcomed the move as an important step toward a comprehensive national policy on medical marijuana because it will allow states to implement their own laws without fear of interference from the federal government.

In all, 13 states have some form of medical-marijuana laws. But some, like New Mexico, Rhode Island, and Michigan, have been reluctant to implement programs amid fears they would be struck down by courts or shut by authorities, said Graham Boyd, director of the ACLU's California-based Drug Law Reform Project.

In New Jersey, the state Senate approved a medical-marijuana bill in February, with bipartisan support. An amended version, tightened to address concerns that marijuana could become too readily available, awaits a floor vote in the state Assembly. If approved, it would return to the Senate for a second vote. Gov. Corzine has said he would sign the bill if the Legislature approves it.

Under the amended bill, the state would issue identification cards to patients diagnosed with a "debilitating medical condition." Those patients would be permitted to obtain marijuana from authorized nonprofit alternative-treatment centers, in person or via courier or delivery.

Roseanne Scotti, director of Drug Policy Alliance New Jersey, which favors medical marijuana, said of yesterday's Justice Department move: "We're thrilled.. . . We think it bodes very well for the future of medical marijuana in New Jersey."

Attorney General Eric H. Holder Jr. said the new guidelines were adopted, in part, because federal law enforcement agencies had limited resources and need them for more pressing priorities. One of those priorities is countering the spread of violent Mexican drug cartels, which use the vast profits from their marijuana sales in the United States to support other crimes, the guidelines say.

The new guidelines stress that authorities should go after those who improperly abuse or obtain medical marijuana or use clinics as a cover for drug dealing and other illegal activity.

In particular, the Justice memo urges authorities to pursue cases that involve violence, illegal use of firearms, sale of marijuana to minors, excessive financial gains, and ties to criminal enterprises.

The change in policy was criticized by many law enforcement advocates, and some conservative groups and members of Congress.

Rep. Lamar Smith (R., Texas), ranking member of the House Judiciary Committee, said the decision undermined the administration's get-tough plan to attack the Mexican cartels, which he said were fueling drug-related violence along the U.S.-Mexico border.