Just look for the Farmacia. They should have a good selection of the older, standard drugs like zithromax, penicillin, diclofenac, prednisone, albuterol, etc. for very reasonable prices. If you bring your prescription bottle, they should be able to look up the comparable drug for you. As long as your purchase is for personal use, it's a good reason to stock up. Oh, and often there are folks outside hawking the sale of xanax, cialis, viagra, etc.
Just go to one that's clean and tidy. They should be very helpful, courteous and professional. Many will be able to speak English, too.
One of my friends, a respiratory therapist, was visiting a couple of years ago when I bought some medicine in Mexico. Later she was in Peru with family and one them had breathing troubles. Linda remembered our experience at the mexican farmacia, found one in Peru and was able to get what she needed without any problem.