"It is ‘impossible for state law enforcement officers to comply with their state law duty not to forfeit medical marijuana, and their federal law duty not to distribute or aid in the distribution of marijuana," Schuette's opinion said.

In such circumstances, the state law, approved by Michigan voters in 2008, must defer to the federal law, Schuette said.

"The people of this State, even in the exercise of their constitutional right to initiate legislation, cannot require law enforcement to violate federal law by mandating the return of marihuana to registered patients or caregivers," Schuette wrote.

Schuette's opinion was prompted by a request from state Representative Kevin Cotter. Cotter asked whether police were required to return marijuana to medical marijuana patients who had been arrested or detained upon their release.

Medical marijuana advocates said police in Michigan have followed the letter and spirit of the state law by routinely returning marijuana to patients who have been temporarily detained.

The Michigan medical marijuana statute explicitly bars the seizure of marijuana and marijuana paraphernalia from a licensed patient.About 100,000 people are registered to use marijuana for medical reasons in Michigan.

Schuette's opinion does not authorize police to prosecute patients for marijuana possession. It just lets cops know they could face a trafficking charge for enforcing the law.

An opinion from the attorney general is usually treated as a binding legal opinion, especially on state agencies like the Michigan State Police unless and until a judge rejects it.

A California court dealt with the legal question addressed by the attorney general's opinion in 2007. In that case, a medical marijuana patient successfully sued police in Garden Grove, Calif., for the return of marijuana that had been taken from him during a traffic stop. The case was appealed to the U.S. Supreme Court, but the justices declined to hear the appeal.

In his opinion, Schuette cited a 2010 decision by the Oregon Supreme Court. In that case, the state court upheld the right of an employer to refuse to hire a medical marijuana patient on the grounds that using the drug was a violation of federal law.

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