Go directly to a “Farmacia de 1a Clase” (they have stricter requirements) and avoid touts (including taxi drivers) because they get a cut of the higher prices you’ll get charged - and could take you to a dodgy pharmacy with a higher possibilities of counterfeit drugs. Some first class pharmacies even have a doc who can prescribe your meds.

I don’t know how reliable this blogger is, but check this page out.

See this article on Gringo Gazette.