DETROIT (AP) – Medical marijuana became legal in Michigan on Dec. 4, but smoking a joint could still get patients arrested because the regulations needed to protect them won’t be ready for months.

The law approved by voters in November allows patients with cancer, HIV, AIDS, glaucoma and other diseases to use marijuana to relieve their symptoms on a doctor’s recommendation.

Qualifying patients can register with the state and receive ID cards allowing them to legally acquire, possess, grow, transport and use a limited amount – no more than 2.5 ounces and 12 plants – of marijuana. They also can designate a primary caregiver to receive similar protection.

But those cards won’t be issued until the Department of Community Health introduces guidelines addressing how applications will be handled, what fees will be charged and other issues. The rules must be finalized by April 4.

Until then, anyone possessing marijuana – even patients who could later qualify for the program – can be arrested and prosecuted, though the law allows patients to use a medical-justification defense at trial.

“We have this void where this takes effect now, but there are no rules, regulations or guidance for the people who want to use it or the people who enforce the laws,” said Jim Valentine, chief of police in Lowell and first vice president of the Michigan Association of Chiefs of Police.

Officers in Lowell will arrest marijuana users even if they claim to be patients awaiting cards, Valentine said. He said he’ll let the prosecutor decide whether to pursue charges.

A medical-marijuana program nearly identical to Michigan’s was implemented without major incident in Rhode Island in 2006, said Charles Alexandre, who oversees the program as chief of health professions regulation in Rhode Island’s Department of Health. That state also had a period where the law went into effect before the regulations were in place, and patients simply had to wait until the rules were in order.

Michigan is the 13th state to allow medicinal use of marijuana, though the state’s law doesn’t address how patients can obtain it. It’s illegal to sell marijuana, even to registered patients. That’s also the case in several other states.

Police in Michigan say they want guidance on the issue, and some experts said the Legislature may have to intervene if that or any other aspect of the program becomes a problem.