Ivermectin is safe for pregnant mares. The FDA-approved label for ivermectin paste explicitly states it may be used in horses of all ages, including mares at any stage of pregnancy, without adversely affecting fertility in stallions. This makes ivermectin one of the most widely trusted dewormers for use throughout a mare’s gestation.
What the Research Shows
A controlled study published in ScienceDirect evaluated ivermectin administered to pregnant mares at 60-day intervals all the way up to and including the date of foaling. None of the mares in the study showed any adverse clinical signs during the entire course of treatment, and all 20 mares delivered live, healthy foals that thrived through their yearling year. The researchers concluded that ivermectin is both safe and highly effective when given to pregnant mares.
This aligns with what veterinarians have observed in decades of field use. Ivermectin has been a standard equine dewormer since the 1980s, and reproductive problems linked to its use in mares have not materialized as a clinical concern.
The First 60 Days of Pregnancy
There is one general precaution worth knowing. The Merck Veterinary Manual recommends that anthelmintics (dewormers of any kind) should not be administered to mares during the first 60 days of gestation. This window corresponds to organogenesis, the period when the embryo’s major organs are forming and is most vulnerable to outside influences. This is a broad precaution that applies to all dewormers, not a specific warning about ivermectin.
If your mare was recently bred and you’re unsure whether she’s within that early window, it’s reasonable to hold off on deworming until after day 60. After that point, ivermectin can be used on its normal schedule throughout the rest of pregnancy without concern.
Deworming Before Foaling
One of the most important times to deworm a pregnant mare is right before she foals. The Merck Veterinary Manual specifically recommends administering ivermectin or a benzimidazole one to three days before the expected foaling date. The reason is a parasite called Strongyloides westeri, a threadworm that can pass from mare to foal through milk. Treating the mare just before delivery reduces the parasite load she transmits during nursing, helping prevent diarrhea in the newborn foal during its first weeks of life.
This pre-foaling dose is a standard part of broodmare management on most breeding farms. Many operations also deworm mares roughly one month before the expected foaling date as part of their routine protocol.
Dosage for Pregnant Mares
The standard dose of ivermectin for horses is 200 mcg/kg of body weight, given orally as a paste. This dose does not change for pregnant mares. You administer the same amount based on the mare’s weight as you would for any other adult horse. The study that confirmed safety in pregnant mares actually used a higher dose (600 mcg/kg, given by injection) at 60-day intervals without any adverse effects, which provides a significant margin of safety above the normal oral dose.
As with any horse, estimate body weight carefully. Underdosing contributes to parasite resistance over time, while significant overdosing is unnecessary. A weight tape gives a reasonable estimate, though a livestock scale is more accurate if available.
Safety for the Nursing Foal
Ivermectin does pass into mare’s milk in small amounts, but this has not been shown to cause problems for nursing foals. In fact, treating mares with ivermectin in late pregnancy and around foaling is standard veterinary practice precisely because of the benefits it provides to the foal by reducing parasite transmission through colostrum and milk. Breeding farms routinely administer ivermectin to mares in the pre-foaling period without reports of adverse effects in their foals.
Choosing the Right Product
Stick with plain ivermectin paste rather than combination products unless your veterinarian specifically recommends otherwise. Some combination dewormers pair ivermectin with praziquantel (for tapeworms), which is also considered safe in pregnant mares. However, other dewormer classes have different safety profiles during pregnancy, so it’s worth confirming that any combination product you’re considering has been cleared for use in breeding animals. The product label will state whether it’s approved for pregnant mares.
Your deworming schedule should ideally be based on fecal egg counts rather than a fixed calendar rotation. This approach, sometimes called targeted deworming, helps you treat only when parasite burdens are actually high and reduces the development of drug-resistant worm populations on your farm. A fecal egg count before and after treatment also tells you whether the ivermectin is still effective against the parasites on your property, since resistance to ivermectin has been documented in some small strongyle populations.