We recommend monthly year round flea/tick and heartworm prevention. Products purchased directly from your veterinarian come with a manufacturers' guarantee, meaning the company will stand by the quality of their product and refund or replace the product if it not working. Additionally, if there is proof that your pet has been on consistent monthly heartworm preventative and he comes down with heartworm disease, they will pay a portion of the cost to treat the disease. Diversion occurs when medications are sold to third parties, often online pharmacies for redistribution. Medications purchased outside the veterinary hospital void the manufacturer's guarantee as the manufacturer cannot attest to the authenticity, sourcing or quality of the medications. Improper storage (excess heat) can inactivate or decrease the efficacy. Unauthorized vendors may be sourcing counterfeit products. Additionally our prices are comparable to or often better than online pharmacies. Read an FDA article about online pharmacies here and an article about diversion here.