Fourteen states allow some use of marijuana for medical purposes: Alaska, California, Colorado, Hawaii, Maine, Maryland, Michigan, Montana, Nevada, New Mexico, Oregon, Rhode Island, Vermont, and Washington.
California is unique for the presence of dispensaries - businesses that sell marijuana and even advertise their services.
Attorney General Eric Holder said in March that he wanted federal law enforcement officials to pursue those who violate both federal and state law, but it has not been clear how that goal would be put into practice.
A 3-page memo spelling out the policy is expected to be sent today to federal prosecutors in the 14 states, and also to top officials at the FBI and the Drug Enforcement Administration.
The memo, the officials said, emphasizes that prosecutors have wide discretion in choosing which cases to pursue, and says it is not a good use of federal manpower to prosecute those who are without a doubt in compliance with state law.
The officials spoke on condition of anonymity because they were not authorized to discuss the legal guidance before it is issued.
At the same time, the officials said, the government will still prosecute those who use medical marijuana as a cover for other illegal activity. The memo particularly warns that some suspects may hide drug dealing or other crimes behind a medical marijuana business.
The memo, officials said, is designed to give a sense of prosecutorial priorities to US attorneys in the states that allow medical marijuana.
