The law already allows medical patients to buy up to 2.5 ounces of marijuana every two weeks, rather than the one ounce allowed at any one time to recreational customers. But because of shortages, shops have been setting their own limits on purchases, such as one-eighth an ounce of flower, the smokable part of the plant. Under the new guidelines, shops must let medical patients buy 2.5 times as much as any limit they set for recreational customers.