Ivermectin at standard heartworm-prevention doses is generally safe for pregnant dogs. FDA testing found no teratogenic effects (birth defects) and no impact on the number of implants, resorptions, or live and dead fetuses in treated pregnant dogs compared to untreated controls. That said, dose level and your dog’s breed matter significantly, and there are situations where extra caution or an alternative medication makes more sense.
What FDA Testing Found
Before ivermectin was approved for use in dogs, the FDA required teratology and reproductive safety studies. In one study, pregnant dogs received ivermectin and were compared to a control group that received no active drug. Researchers examined the fetuses for external, visceral, and skeletal abnormalities and found no evidence that ivermectin caused birth defects at any level of examination.
A separate safety study in breeding and pregnant dogs did find congenital abnormalities in a small number of puppies, but four of those puppies came from the untreated control group while only two came from the ivermectin group. The researchers concluded that none of the abnormalities were related to ivermectin treatment, since they occurred at equal or higher rates in dogs that never received the drug. In the male breeding study, the only notable birth defects (an undershot jaw and a cleft palate) appeared in the control group’s puppies, not in the ivermectin group.
The MDR1 Gene and Breed-Specific Risk
The biggest safety variable with ivermectin isn’t pregnancy itself. It’s genetics. Some dogs carry a variant of the MDR1 gene that prevents their bodies from pumping certain drugs out of the brain effectively. When these dogs receive ivermectin, especially at higher doses, the drug can accumulate in the central nervous system and cause serious toxicity: tremors, disorientation, blindness, seizures, and in severe cases, death.
Breeds most commonly affected include Collies, Australian Shepherds, American Shepherds, German Shepherds, Shetland Sheepdogs, and Old English Sheepdogs. If your pregnant dog belongs to one of these breeds and hasn’t been tested for the MDR1 variant, that’s worth addressing before giving ivermectin at any dose. A simple DNA test can confirm her status.
The important nuance: heartworm preventatives use very low doses of ivermectin, and these remain safe even for MDR1-sensitive dogs at FDA-approved levels. The risk climbs when ivermectin is used at the much higher doses sometimes prescribed for mange or other parasitic infections. For a pregnant dog with the MDR1 variant, those higher doses should be avoided entirely.
Ivermectin During Pregnancy for Parasite Control
Beyond heartworm prevention, some breeders and veterinarians use ivermectin strategically during pregnancy to reduce roundworm transmission to puppies. Roundworm larvae lie dormant in a dog’s tissues and reactivate during pregnancy and lactation. About 98% of the larvae that reach puppies do so through the placenta before birth, with roughly 1.5% passing through the milk after birth. Timed ivermectin treatments during pregnancy can interrupt this cycle and produce healthier litters with lower worm burdens.
This strategic deworming approach has been studied in greyhound breeding programs, where reducing roundworm transmission early results in stronger puppies and a cleaner whelping environment. The protocol requires specific timing during gestation, so it’s not something to improvise on your own.
Alternative Preventatives for Pregnant Dogs
If you’d rather avoid ivermectin during your dog’s pregnancy, whether due to breed concerns or personal preference, several alternatives have safety profiles suitable for breeding dogs.
- Selamectin: A topical preventative used by some breeders year-round on their breeding females. It covers heartworm, fleas, and certain intestinal parasites.
- Milbemycin: Another heartworm preventative sometimes combined with lufenuron (a flea control compound). Lufenuron has been found safe for use during pregnancy, making this combination a practical option for pregnant dogs that also need flea protection.
Both alternatives belong to the same broad drug family as ivermectin (macrocyclic lactones, in the case of selamectin and milbemycin), so MDR1-sensitive dogs still need appropriate dose management with these products. However, they offer flexibility if your veterinarian prefers a different active ingredient for your specific dog.
Practical Takeaways for Breeders
If your pregnant dog is already on a monthly heartworm preventative containing ivermectin at standard doses, the FDA data supports continuing it through pregnancy. The reproductive safety studies showed no increase in fetal loss, no increase in birth defects, and no adverse effects on litter size compared to untreated dogs.
Where caution matters most is dose escalation. Using ivermectin at therapeutic levels for conditions like demodectic mange is a very different situation than a monthly heartworm chew, and the safety margin narrows considerably during pregnancy, particularly for MDR1-affected breeds. If your pregnant dog needs treatment for a condition that requires higher ivermectin doses, your veterinarian can recommend a safer alternative or confirm her MDR1 status before proceeding.