There is no single “morning after pill” for dogs the way Plan B works for humans, but veterinarians do have several effective options for preventing or ending an unwanted canine pregnancy. None of them are over-the-counter, and none should be attempted at home. The approach your vet recommends will depend on how recently the mating happened, whether pregnancy is confirmed, and your dog’s overall health.
Why Human Plan B Won’t Work for Dogs
Human emergency contraceptives contain a synthetic hormone designed for human reproductive biology. Dogs process hormones differently, and their reproductive cycle is fundamentally unlike ours. A dog’s fertile window can last over a week, sperm can survive in the reproductive tract for days, and the hormonal signals that maintain pregnancy differ from those in humans. Giving your dog a human morning-after pill will not reliably prevent pregnancy and could cause harm, particularly in intact (unspayed) females who are more vulnerable to estrogen-related side effects.
The “Mismate Shot” and Why Vets Stopped Using It
For decades, the standard emergency treatment was an estrogen injection, commonly called the “mismate shot,” given shortly after an unplanned breeding. It was highly effective, preventing pregnancy roughly 95% of the time. But it came with serious risks that eventually made most veterinarians abandon it.
Estrogen injections can cause a life-threatening suppression of bone marrow, the tissue that produces red blood cells, white blood cells, and platelets. Because the drug can damage stem cells, this bone marrow toxicity is sometimes irreversible. On top of that, about 25% of dogs given the shot developed pyometra, a dangerous uterine infection that often requires emergency surgery. The Merck Veterinary Manual now explicitly advises against using estrogen-based treatments to prevent pregnancy in dogs.
What Vets Use Instead
Modern veterinary medicine offers safer alternatives, though all require a prescription and veterinary supervision.
Aglepristone (Alizin)
This is the closest thing to a canine morning-after pill. Aglepristone works by blocking progesterone, the hormone dogs need to establish and maintain pregnancy. It’s given as two injections 24 hours apart and can be used up to 45 days after mating. In field trials, it was effective in about 95% of confirmed pregnancies, with only around 5% of cases resulting in partial rather than complete termination. It’s widely available in the UK and parts of Europe but is not currently approved in the United States, which limits access for American dog owners.
Prostaglandin-Based Treatments
Prostaglandins are hormones that cause the uterus to contract and can end a pregnancy by breaking down the structures that support it. These are given as a series of injections over several days, and newer synthetic versions target the uterus more precisely, causing fewer side effects than older formulations. Combination protocols that pair a prostaglandin with a drug that suppresses the hormone prolactin have shown the highest success rates with the fewest side effects and shortest treatment times, though they can be more expensive.
Corticosteroid Protocols
Oral corticosteroids given twice daily can also reliably terminate canine pregnancy. This option is typically used later in gestation and requires ongoing doses until the pregnancy ends, with ultrasound monitoring to confirm the outcome.
Your Dog May Not Even Be Pregnant
Before rushing into any treatment, it’s worth knowing that an observed mating does not guarantee pregnancy. When researchers looked at dogs brought in for pregnancy termination after an unplanned breeding, more than 60% turned out not to be pregnant at all when checked by ultrasound. Dogs can mate without successful fertilization for many reasons: the timing may have been off within the heat cycle, the male may not have achieved a full tie, or the breeding may not have been as complete as it appeared.
Pregnancy in dogs can be confirmed with a blood test that detects a hormone called relaxin as early as 22 days after mating, though results are more reliable after 28 days. Ultrasound can also confirm pregnancy around this time. Because more than half of “mismated” dogs aren’t actually pregnant, many vets recommend waiting to confirm before starting any drug treatment. Unnecessary medication exposes your dog to side effects for a problem that may not exist.
Timing Matters
The window for intervention is wider than you might expect. Unlike human emergency contraception, which works best within 72 hours, canine options can be effective weeks after mating. Aglepristone can be used up to 45 days post-mating. Prostaglandin protocols are typically started after pregnancy is confirmed by ultrasound, usually around day 25 to 30. This means you generally have time to confirm whether your dog is actually pregnant before committing to treatment.
That said, earlier intervention tends to be simpler and carries fewer risks. If you know your dog mated and you want to prevent a litter, contact your vet within the first few days. They can advise you on whether to treat immediately or wait for a pregnancy test based on your dog’s specific situation.
Spaying as a Definitive Option
If you don’t plan to breed your dog in the future, spaying is the most straightforward solution. It can be performed during early pregnancy and eliminates both the current pregnancy and any chance of future ones. It also removes the risk of pyometra entirely. For many owners dealing with an accidental mating, spaying ends up being the most practical and cost-effective choice, especially compared to multiple vet visits for drug-based protocols that require ultrasound monitoring.
What Not to Do
Do not give your dog any human medications, herbal supplements, or home remedies to try to end a pregnancy. Human birth control pills, even in large quantities, won’t reliably prevent canine pregnancy and can cause estrogen poisoning in intact females. There are no safe over-the-counter options for dogs. Every effective treatment requires veterinary involvement, both for the prescription itself and for the monitoring needed to confirm the pregnancy has been fully terminated.