Federal prosecutions for the use of "medical marijuana" in states where the substance is permitted by law for the treatment of illness will be eased, according to the US Justice Department.
Marijuana use is a federal crime in the United States, but 14 US states, including California, allow use of the drug to alleviate medical symptoms.
Attorney General Eric Holder said that in the future, federal law enforcement resources will not be used to prosecute patients whose state laws allow prescription use of the drug, in a sharp departure from the policy of the George W. Bush administration.
"It will not be a priority to use federal resources to prosecute patients with serious illnesses or their caregivers who are complying with state laws on medical marijuana," said Holder in a statement.
He added however that "we will not tolerate drug traffickers who hide behind claims of compliance with state law to mask activities that are clearly illegal."
"This balanced policy formalizes a sensible approach that the department has been following since January," said Holder, the top federal prosecutor.
He added that the new rules "effectively focus our resources on serious drug traffickers while taking into account state and local laws."
States that allow some use of marijuana for medical purposes are Alaska, California, Colorado, Hawaii, Maine, Maryland, Michigan, Montana, Nevada, New Mexico, Oregon, Rhode Island, Vermont and Washington.
The new guidelines, which were sent to federal prosecutors across the United States, codify a policy that the government has followed since President Barack Obama took office in January, Holder said.
On the campaign trail while vying for the White House last year, Obama promised that he would not prosecute medical marijuana users in states where the practice was legal under state law.