Yes, there is a veterinary equivalent of Plan B for dogs. It’s commonly called a “mismate injection,” and it works by blocking the hormone that sustains pregnancy. Unlike human Plan B, it’s not a pill you can buy over the counter. It requires a vet visit and a course of two injections given 24 hours apart.
How the Mismate Injection Works
The active drug is a synthetic compound that binds to progesterone receptors with three times the strength of progesterone itself, but without producing progesterone’s effects. Progesterone is the hormone that maintains pregnancy in dogs, so blocking it effectively shuts the process down. When given early (before about 24 days after ovulation), it prevents the embryos from implanting. Between days 25 and 35, it causes the embryos to be reabsorbed by the body. After day 35, it triggers expulsion of the fetuses.
The treatment can be given at any point after the unwanted mating, up to 45 days afterward. However, it’s more effective and causes fewer side effects when given within 21 days of mating.
What to Expect at the Vet
Your dog will need two visits, one day apart. The vet administers an injection at each visit. Because the appointments must be exactly 24 hours apart, most clinics won’t start the course on a Saturday.
Before treatment, your vet may want to confirm whether your dog is actually pregnant. A blood test can detect a pregnancy hormone as early as 22 to 27 days after breeding, though testing too early can produce a false negative. If that happens, the test is typically repeated a week later. Ultrasound can pick up developing embryos as early as three weeks post-breeding, and a physical exam by an experienced vet (feeling the abdomen for uterine swellings) works best between three and a half to five weeks. Many dogs brought in after an unwanted mating turn out not to be pregnant at all, so confirming pregnancy first can save you the cost and your dog the discomfort of unnecessary treatment.
Cost and Availability
This isn’t cheap. Pricing from a UK veterinary hospital puts the course at roughly £390 to £655 (approximately $500 to $830 USD) depending on your dog’s weight, since larger dogs need higher doses. Prices vary by clinic and country. The drug is widely available through veterinary practices in Europe, though availability in the United States is more limited. Your vet can advise on what options are accessible in your area.
Why Older “Mismate Shots” Fell Out of Favor
For decades, vets used estrogen-based injections to prevent pregnancy after an unwanted mating. These are no longer recommended. Estrogen can cause bone marrow toxicity in dogs, a condition that progresses through stages: first a dangerous drop in platelets, then abnormal white blood cell changes, and finally either recovery or complete bone marrow failure. The response is unpredictable. Some dogs given the same dose experience only mild effects while others develop fatal suppression. Estrogen injections also increase the risk of a serious uterine infection called pyometra. No safe and effective dose was ever established, which is why the newer progesterone-blocking injection replaced them.
If a vet offers an estrogen-based mismate shot, that’s a red flag. The current standard of care uses the progesterone-receptor blocker.
Don’t Give Your Dog Human Plan B
Human emergency contraception uses a completely different hormone (a synthetic progestin called levonorgestrel) that works through a different mechanism than what’s needed in dogs. While long-term toxicology studies have used levonorgestrel in dogs at research doses without acute poisoning, it was never designed or tested as a canine contraceptive. It won’t reliably prevent pregnancy in dogs, and giving your dog human medication without veterinary guidance risks unexpected side effects. The canine reproductive cycle is fundamentally different from the human one, so what works for people simply doesn’t translate.
Spaying as a Permanent Solution
If your dog has had one unplanned mating, the most reliable way to prevent it from happening again is spaying. In some cases, a vet can perform a spay even during early pregnancy, which both ends the pregnancy and permanently prevents future ones. This is worth discussing with your vet, especially if you weren’t planning to breed your dog. The mismate injection is effective as an emergency measure, but spaying eliminates the risk entirely and also removes the chance of uterine infections and reduces the risk of mammary tumors later in life.