Now Accepting Debit Cards! Adults 21 and older may purchase up to 1 ounce, or 28.35 grams of marijuana, with a valid driver’s license, state or federal ID. All adult-use purchase are subject to a 15% cannabis excise tax, an 8.5% San Francisco city sales tax. To buy Medicinal Cannabis, you must be 18 or older and have either have current physician’s recommendation, a valid county-issued medical marijuana identification card. Adults 18 and older with a valid medical marijuana identification card can purchase up to 8 ounces, or 226.8 grams, of dried cannabis or concentrates. Medical patient are not subject to pay the sales and when making retail purchases of medical cannabis, concentrates, edibles, or topical products.