If your dog just mated unexpectedly, pregnancy can still be prevented, but you need to act quickly. The most effective option is a veterinary injection that blocks pregnancy hormones, and it works best when given within the first few days after mating. There is no safe at-home method to prevent dog pregnancy after the fact.
Contact Your Vet Within 24 Hours
Time matters. The sooner your vet can intervene, the simpler and safer the process. Call your veterinary clinic as soon as you realize mating has occurred, even if it happened overnight or over the weekend. Many emergency veterinary clinics can handle this. Let them know approximately when the mating took place, whether your dog is in heat, and whether she’s been bred before.
Don’t attempt any home remedies, herbal treatments, or human medications. None of these are effective, and some can be toxic to dogs.
The Standard Treatment: A Hormone-Blocking Injection
The most widely used and safest treatment is a drug called aglepristone (sold as Alizin in many countries). It works by blocking progesterone, the hormone dogs need to establish and maintain pregnancy. Without progesterone doing its job, embryos cannot implant in the uterine wall.
The treatment involves two injections given 24 hours apart. When administered within the first 25 days after mating, aglepristone prevents pregnancy in virtually 100% of cases. Even at reduced doses, studies have shown efficacy rates above 91%. The earlier it’s given, the more straightforward the outcome: the embryos simply fail to implant, and the body reabsorbs them without any visible signs.
If treatment happens later, between 25 and 35 days after mating, the embryos are reabsorbed in a similar way but the process takes longer. After day 35, the pregnancy is far enough along that the body expels the fetuses, which is more physically taxing on your dog. This is why early treatment is strongly preferred.
Side effects from aglepristone are generally mild. Some dogs experience temporary soreness at the injection site or mild lethargy. Serious complications are rare, which is a major reason this drug replaced older treatments.
Availability Varies by Country
Aglepristone is widely available in Europe, Australia, and many other countries, but it is not currently approved in the United States. If you’re in the U.S., your vet will discuss alternative hormonal protocols that may involve prostaglandins or other medications. These can be effective but often require more visits and closer monitoring. Ask your vet directly about what’s available in your area.
Why the Old “Mismating Shot” Is No Longer Used
For decades, vets gave estrogen injections (typically estradiol) after accidental matings. This practice has been abandoned. Estrogen-based mismating shots carry serious risks, including life-threatening bone marrow suppression, uterine infections (pyometra), and prolonged heat cycles. No safe or effective dose was ever established, and fatal reactions have been documented.
Modern veterinary guidelines are clear: estrogen injections should not be used for mismating. If a vet suggests this approach, it’s worth seeking a second opinion. Aglepristone and other current options are far safer and more effective.
Emergency Spay as an Alternative
If you never plan to breed your dog, an emergency spay (ovariohysterectomy) is another option. This surgery removes both ovaries and the uterus, permanently preventing pregnancy and eliminating future heat cycles. Any developing embryos are removed along with the uterus.
There’s a timing consideration, though. Vets typically prefer to spay two to three months after a heat cycle, when hormone levels have settled and the blood supply to the reproductive organs is less engorged. Spaying during or shortly after heat carries a higher risk of surgical bleeding and complications. That said, the risks of an emergency spay are still manageable for most healthy dogs, and for many owners, the benefits of permanent sterilization outweigh the slightly elevated surgical risk.
Discuss your dog’s age, breed, and overall health with your vet to decide whether an emergency spay or a hormone-blocking injection makes more sense for your situation.
Confirming the Treatment Worked
Regardless of which treatment your dog receives, follow-up is essential. A single vet visit isn’t enough to confirm success.
Pregnancy in dogs can be detected by ultrasound as early as three weeks after mating. A blood test measuring a hormone called relaxin, which is produced by a developing placenta, can confirm pregnancy between 22 and 27 days post-breeding. Your vet will likely recommend one or both of these tests around days 28 to 30 after the mating to verify that your dog is not pregnant.
Some vets also schedule a second check around the midpoint of what would have been the pregnancy (roughly day 45) to be absolutely certain. If pregnancy is detected at that point, further treatment or allowing the pregnancy to continue can still be discussed. Dog pregnancies last about 63 days, so there is a window for decisions, but earlier detection gives you more options.
What to Watch For at Home
After treatment, keep an eye on your dog for a few days. Mild vaginal discharge is normal and usually resolves on its own. Decreased appetite or slight lethargy for a day or two is also common. Contact your vet if you notice heavy bleeding, persistent vomiting, fever, or if your dog seems unusually lethargic for more than 48 hours.
It’s also worth knowing that a dog who has been treated can still get pregnant again during the same heat cycle if she mates a second time. Keep her separated from intact males until her heat is completely finished, which can take one to two weeks after the mating incident. Signs that heat is ending include reduced vulvar swelling and a return to normal behavior around other dogs.
Preventing Future Accidental Matings
Spaying remains the most reliable way to prevent unwanted pregnancies permanently. If you’re not planning to breed your dog, spaying eliminates the risk entirely and also reduces the likelihood of mammary tumors and uterine infections later in life.
If you choose not to spay, managing heat cycles requires vigilance. Female dogs typically go into heat twice a year, and the fertile window lasts roughly 5 to 10 days within a heat cycle that spans about three weeks total. During this time, male dogs can detect a female in heat from a surprising distance. Secure fencing, leashed walks only, and keeping your dog indoors when unsupervised are the practical essentials. Even brief lapses in supervision can result in mating, as the process can happen in minutes.