You can get over the counter medicines at any pharmacy in the tourist zone. The quantity, prices, and availability of certain pharmaceuticals may vary at locations; usually higher prices near better residencials, lower prices around the barrios. The pharmacies will sell by the pill, but check the expiration date on the carton before accepting. It is good to know the generic or chemical name of certain medicines especially if you have purchased them in your home country under a brand name. Brand names you are familiar with may not be sold here. Drug products similar to the brand names are available.
PJT has found the pharmacy at Central Medico Punta Cana on Av Espana in Bavaro is a good local source of medicines.
If you wish to go outside the tourist zone you can find much lower prices at the "Boticas", Farmacias del Pueblo. There is one in La Otra Banda, on the main road, opposite the Policia National and the other at the end of the street of Public Hospital, Nuestra Senora de la Altagracia in Higuey. Please note: their prices are extremely low however at times there is limited availability of certain medicines.
Regards,
PJT