There are lots of issues with online pharmacies, and I won't go into all of them here. The main issue is that the companies who manufacture these products (Novartis, Merial, Pfizer) only sell their products to licensed veterinarians. They do this so they will have some sort of control over how their product is used, shipped, stored, and labeled.
These companies will guarantee their products, and if your dog gets heartworms or intestinal worms (and in some cases, fleas) they will pay for the treatment. If the product is bought from an online pharmacy, this guarantee is voided.
In order for online pharmacies to obtain these products to sell, they have to buy through diversion of product. In most cases, they offer large sums of money to a vet to buy a large amount of product in bulk and resell it to them. This is unethical, and in some cases, illegal.
There are some real concerns about products obtained from online pharmacies. I have had clients receive expired products, products labeled in foreign languages, products in opened packaging, etc. Who knows how this product has been stored or how it is obtained?
The vast majority of the time, the product would be just fine, but you are taking a small risk.
At my clinic, this is our policy:
I will not refuse to sign a prescription for a heartworm medication (unless there is a medical reason), but I will speak to the client first to let them know my concerns. I also offer to match the price if it is cheaper than my price (often, it is not). If they still wish to purchase this product, I oblige.
In only one case will I refuse a prescription. If a client orders from 1-800-Pet Meds, I WILL NOT sign it. On more than one occasion, they have forged my signature and/or sent products to my client after it was refused for medical reasons. They are dirty and I refuse to deal with them.
I do require a heartworm test once a year, or I require a signed release if the test is declined. This is because overall about 90% of dogs in the Houston area will get heartworms if they are not on preventive. A recent study showed that up to 70% of dog owners have been late or missed a scheduled dose of heartworm prevention in the past 6 months. These medications are not perfect even when used as directed, but late and missing doses increases the chance of contracting heartworms. In some cases, giving a heartworm preventive to a dog with heartworms can cause a serious, shock-like reaction.
I would love to be able to have all my clients buy online if it was controlled by the manufacturer and they had to follow my rules. I make no money on heartworm or flea products as it is, and would just as soon not have tens of thousands of dollars worth of these meds sitting on my shelf. My overhead would be cut by 20% if I did not carry any heartworm or flea products.