Some pet medications require a prescription and others can be purchased over the counter (OTC). OTC pet medications can be purchased through online pet pharmacies, pet-supply stores and big-box retail stores. It is not safe to attempt to purchase prescription pet medications without a valid prescription from a veterinarian who has seen your pet. Companies that offer prescription medications without a prescription are breaking the law. Often, products sold this way are counterfeit or expired. Additionally, it is not safe to give your pet medications that were not prescribed by your veterinarian.