Email Sign-Up Go to Shopping Cart (0)
Drs. Foster and Smith Pet Supplies

Free Shipping on orders over $49

Customer Service

Heartworm Preventive FAQs

Drs. Foster & Smith Educational Staff
Dewormer Comparison Chart 
Dog Deworming Guidelines 
Mosquito Control 
Heartworm Prevention in Dogs

Why do I need a prescription for heartworm preventives?
By federal law, a doctor-patient relationship must be established (your veterinarian-your dog) and a prescription is required for a heartworm preventive. Your veterinarian needs to determine your dog's heartworm status before she writes a prescription for heartworm preventives. If you give a dog with heartworm disease any heartworm preventive, this could be harmful to the dog.

Can I use any natural preventives for heartworm?
There are no natural preventives for heartworm that have been shown to be truly effective.

How do monthly heartworm preventives work?
Monthly medications are given every 30 days. Monthly heartworm preventives interfere with the development of the immature heartworm and prevent it from maturing into an adult. They have a retroactive effect, which means they will eliminate the immature heartworms that have infested the dog in the preceding month. Some pet owners give their heartworm preventives at the beginning of the mosquito season and one month past the end of the mosquito season. We recommend that you give your dog his monthly heartworm preventives year round. There are two reasons we recommend this, even in seasonal areas:

  • To ensure the medicine has been given properly by the pet owner.
  • To prevent intestinal parasites, since some monthly heartworm preventives have activity against intestinal parasites.
Which heartworm preventives can be used to eliminate intestinal parasites? Are these products stronger than other heartworm preventives?
Certain heartworm preventive products, such as Heartgard Plus, Tri-Heart Plus, and Revolution are also effective in removing specific intestinal and/or external parasites and are labeled for these uses. These products are no stronger, and have no more of the active drug (Ivermectin) than products that are not effective against parasites other than heartworm. The preventives that are effective against other parasites either contain an additional ingredient, such as Pyrantel Pamoate, effective against internal parasites, or contain an active ingredient that, by itself, is effective against a number of different parasites - an example is Selemectin in Revolution.
Heartgard PlusHeartgard Plus, a preventive that contains Ivermectin and Pyrantel.
Sentinel ®Sentinel, contains Milbemycin Oxime to prevent heartworm and control some intestinal parasites.
These products meet the same safety requirements as the preventives that are effective against heartworm only.

Do I need to worry if I have forgotten to give my dog his heartworm preventive two months in a row?
Monthly heartworm preventives are effective against the heartworm microfilariae (the immature heartworm) that accumulate in the dog's body in the month prior to giving the dose of preventive. These products also have a retroactive effect of one month, that is, they have an additional month's "safety net" once the owner resumes giving the preventive. This is only a safeguard for someone who has inadvertently forgotten to give one regularly scheduled monthly dose. If an owner forgets to give the monthly heartworm preventive for three or more months, we recommend that the dosing be resumed immediately and the dog tested for possible heartworm infection seven months to a year after resumption. A heartworm test can only detect the presence of the heartworm parasite after it is at least 7 months old.

For additional information, view our "How to Manage Heartworm" video.


Click here for a pdf version of this article.  



Contact us