Oakland attracted national attention last week with the country’s first voter-approved medical marijuana business tax — a 15-fold increase that won easy victory in a special election Tuesday.

L.A. city attorney and county district attorney say purchasing marijuana at dispensaries is illegal. Officials aim to stamp out ‘greedy’ dispensary owners who buy, rather than grow, their supply.

Los Angeles County Dist. Atty. Steve Cooley said Thursday he will prosecute medical marijuana dispensaries for over-the-counter sales, targeting a practice that has become commonplace under an initiative approved by California voters more than a decade...