If you're looking to save a little money on your flea/tick or heartworm preventative by purchasing a name brand from an online pharmacy or by purchasing a "generic" from another store, BE CAREFUL. Here are a few reasons why you would be better off purchasing a well-known brand from your veterinarian.
1) Products sold through a licensed veterinarian are generally guaranteed through the manufacturer to be safe and effective. If the product does not work for you or your pet is one of the small percentage of animals that has a bad reaction to the product, the company will generally refund your purchase and/or cover the cost of treatment if the product made your pet ill or was ineffective.
2) So-called "generics" often do not contain the same ingredients or the same amounts of active ingredients as the brand-name products, and therefore do not kill and control pests in the same way or as effectively.
3) Reported reactions for over the counter, "generic" products seem to be higher in frequency and more severe in their effect.
4) Over the counter products, if applied incorrectly, can be deadly to your pet. The client-veterinarian relationship helps to ensure that pet owners are properly instructed about dosing and application of flea/tick products, many of which can be deadly if they are ingested, applied at the wrong dosage, or applied to the wrong species (i.e. putting a dog's medication onto a cat).
5) The FDA has "shut down 1677 online pharmacies for selling counterfeit or substandard medication, or for selling drugs without appropriate safegards." (CNN article, below) Licensed veterinarians purchase their medications from reputable distributors and, often, directly from the manufacturer, so you know you're buying the real thing.
6) Most flea/tick and heartworm preventative manufacturers only want their products to be sold through a licensed veterinarian, which means that if the product proves to be ineffective and you purchased it from an online pharmacy, you may have no recourse. For example, if you buy heartworm preventative over the web and your pet comes up positive for heartworms months later, the online pharmacy is not going to help you pay for your pet's treatment, which could run into the $1000s AND have long-lasting implications for your pet's continued health.