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Will heartworm treatment harm unborn puppies in a pregnant dog?
Updated on September 23rd, 2025
We are looking at adopting a 2 yr old dog that has heartworms but is being treated. Looking at her she may be pregnant. Will the heartworm meds harm the unborn puppies? Thank you,
1 Answer
Most Helpful Answer
Answered By Jenna Beyer, DVM, MBA, cVMA
Veterinarian, Certified Veterinary Acupuncturist
Published on August 31st, 2017
There are several medications used in the treatment of heartworms. The "slow kill" method (which is not recommended) involves monthly heartworm preventative. This will be safe for pregnant dogs.
The recommended "fast kill" method usually involves the use of heartworm preventative, doxycycline, prednisone, and injections of melarsomine. Other than the preventative, all 3 medications can cause abortion or congenital defects in pregnant dogs.
If you have not had her examined by a vet, I would recommend doing so as soon as possible. Depending on how far along she could be, ultrasound or x-rays may be used to diagnose pregnancy. If she is pregnant, then heartworm treatment should be placed on hold until the puppies are weaned.
You can't judge pregnancy based on how a dog's belly looks. An enlarged belly could be fluid building up from heart or liver failure caused by damage from the heartworms.
The recommended "fast kill" method usually involves the use of heartworm preventative, doxycycline, prednisone, and injections of melarsomine. Other than the preventative, all 3 medications can cause abortion or congenital defects in pregnant dogs.
If you have not had her examined by a vet, I would recommend doing so as soon as possible. Depending on how far along she could be, ultrasound or x-rays may be used to diagnose pregnancy. If she is pregnant, then heartworm treatment should be placed on hold until the puppies are weaned.
You can't judge pregnancy based on how a dog's belly looks. An enlarged belly could be fluid building up from heart or liver failure caused by damage from the heartworms.
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