ST. PAUL (AP) - State officials are trying to learn why nearly one-seventh of its medical marijuana patients have stopped buying the medicine.

The state's Office of Medical Cannabis told the Associated Press Monday it sent a letter to 92 patients who haven't purchased medical marijuana in 60 days seeking more information. That's nearly one-seventh of the 662 patients registered as of last week.

Price has been a major hurdle for patients in the new medical marijuana program. Monthly costs that can quickly approach hundreds to thousands of dollars have prompted some patients to revert to buying marijuana on the black market.

Medical marijuana went on sale in July. The state's two manufacturers have each implemented a series of discounts to help with costs.