There is no over-the-counter “Plan B” pill for dogs, but veterinary medicine does offer several effective options to prevent or terminate an unwanted pregnancy after mating. The most widely used is an injectable progesterone blocker called aglepristone, which has a 95 to 100 percent success rate and can be given any time from the day of mating up to day 45 of pregnancy. The catch: aglepristone is approved in Europe but not currently available in the United States, so American dog owners have a more limited set of options that a veterinarian must manage directly.
What Aglepristone Does and How It Works
Aglepristone blocks progesterone, the hormone a dog’s body needs to establish and maintain pregnancy. It’s given as two injections under the skin, 24 hours apart. When administered before day 24 after ovulation, it prevents embryos from implanting in the uterus. Between days 25 and 35, the embryos are resorbed by the body. After day 35, the treatment causes expulsion of developed fetuses.
Side effects are uncommon and generally limited to brief pain at the injection site. Some dogs experience temporary mammary swelling, mild vaginal discharge, or decreased appetite. Importantly, the drug does not appear to affect a dog’s ability to become pregnant in the future, making it a good option for breeding dogs involved in an accidental mating.
If you’re outside the United States, in Europe, the UK, or many other countries, this is the standard first-line treatment your vet will likely recommend. It’s marketed under the brand name Alizin.
Options Available in the United States
Because aglepristone isn’t FDA-approved or readily available in the U.S., American veterinarians rely on a different set of drugs to end unwanted canine pregnancies. None of these are as simple as a single pill, and all require veterinary supervision.
Prostaglandins
Prostaglandins work by breaking down the structures on the ovaries that produce progesterone, and by causing the uterus to contract and expel its contents. These are given as multiple injections over several days. Synthetic versions like cloprostenol target the uterus more precisely and cause fewer side effects than natural prostaglandins. Common side effects include excessive drooling, vomiting, diarrhea, and restlessness, though these tend to be dose-related and milder with the synthetic versions. Your dog will typically need to stay at or make repeated visits to the veterinary clinic during treatment.
Prostaglandins Combined With Dopamine Agonists
Veterinarians sometimes pair prostaglandins with a drug called cabergoline, which suppresses prolactin, another hormone that supports pregnancy. In studies, this combination was effective when started around day 25 to 32 after ovulation. In groups given lower doses of the synthetic prostaglandin cloprostenol alongside cabergoline, no adverse reactions were observed, suggesting this combination can be better tolerated than prostaglandins alone.
Dexamethasone
Oral dexamethasone, a corticosteroid, is another option that has been used for pregnancy termination starting around day 30 to 35 of confirmed pregnancy. In a study of 80 pregnant dogs, treatment was successful in 75 cases. The protocol involves twice-daily oral dosing over roughly 7.5 to 9.5 days, with the dose gradually tapered down. Pregnancy termination typically occurred 7 to 15 days after starting treatment. Five dogs in the study retained live puppies after the shorter protocol, so this method isn’t as reliable as aglepristone and may require a second round of treatment.
Why the Old “Mismatch Shot” Is No Longer Recommended
For decades, veterinarians in the U.S. used estrogen injections (estradiol cypionate) as a post-mating treatment. This practice has largely been abandoned, and for good reason. Estrogen can cause life-threatening bone marrow suppression in dogs, a condition where the body stops producing enough blood cells. Dogs with this toxicity develop severe anemia, uncontrolled bleeding, and vulnerability to infection. In untreated cases, the prognosis is poor, and death from hemorrhage or infection is common.
Estrogen injections also carry a significant risk of pyometra, a dangerous uterine infection that causes symptoms like abdominal swelling, vomiting, and excessive thirst, and often requires emergency surgery. On top of all that, most dogs brought in after an accidental mating turn out not to be pregnant in the first place, meaning many dogs were exposed to these serious risks unnecessarily. If a vet still offers an estrogen-based mismatch shot, it’s worth seeking a second opinion.
Timing Matters: When to Act
The sooner you contact your vet after an unwanted mating, the more options are on the table. But you don’t need to panic in the first hour the way you might with human emergency contraception. Dogs have a long window because their reproductive timeline is slower.
Pregnancy in dogs can’t be confirmed by ultrasound until roughly 25 to 28 days after mating. A blood test that detects a pregnancy hormone called relaxin can sometimes pick up pregnancy as early as 22 days, but is more reliable after 28 days. This means your vet may recommend waiting a few weeks before confirming pregnancy, then deciding on a treatment approach. That said, if aglepristone is available in your country, it can be given right away without waiting for confirmation, since it’s safe and effective from day zero onward.
For prostaglandin-based treatments, vets generally wait until pregnancy is confirmed by ultrasound, typically after day 25. Dexamethasone protocols start even later, around day 30 to 35. The further along the pregnancy, the more involved the process becomes, so earlier intervention is generally simpler for both the dog and the owner.
Spaying as a Definitive Option
If you don’t plan to breed your dog in the future, spaying (ovariohysterectomy) is the most straightforward solution after an accidental mating. It ends the pregnancy and permanently prevents future ones. Many vets will perform a spay on a pregnant dog, though the procedure is somewhat more complex than a routine spay due to increased blood supply to the uterus. This is a one-time surgical solution compared to the multi-day drug protocols, and it eliminates the risk of pyometra later in life as well.
What This Costs
There’s no standard price because it depends on your location, the method used, and how far along the pregnancy is. Ultrasound confirmation, blood work, and multiple vet visits for injection-based protocols can add up quickly. Prostaglandin treatments that require hospitalization or frequent monitoring tend to be on the higher end. In countries where aglepristone is available, the two-injection protocol is relatively straightforward and typically less expensive than multi-day prostaglandin regimens. Spaying, while a surgical cost, may be comparable or even cheaper than drawn-out medical protocols, especially if you were already considering it.
The bottom line: there’s no pill you can pick up at a pet store, but effective veterinary options exist. Your vet can help you choose the best approach based on how much time has passed, whether your dog is confirmed pregnant, and whether you want to preserve her ability to breed in the future.