if you talking about the main drag, avenue de revolution, dont even bother, most of the stuff is bunk, or not what they say it is.
If you find a pharmacy outside of the mian tourist area you might have better luck, best bet it to search through old issues of MD they had a huge article about pharmacies in mexico, if i remember correctly they even named names, or at least the city and what was around it.
But yes, everywhere takes credit cards, if you really want them to have your credit card #.