Alizin for Dogs: How Aglepristone Works to Terminate Canine Pregnancy
Alizin for dogs (aglepristone) is a veterinary antiprogestogen used to terminate unwanted pregnancies. Learn the science, dosing, safety, and alternatives.
What Is Alizin for Dogs?
Alizin is the brand name for aglepristone, a synthetic antiprogestogen administered by subcutaneous injection to terminate unwanted pregnancies in dogs. It works by blocking progesterone receptors, causing the uterine lining to break down and ending gestation. Alizin is approved for veterinary use in the European Union, the United Kingdom, Australia, and several other countries, but it is not currently FDA-approved in the United States. In the U.S., veterinarians may use alternative protocols such as prostaglandin-based regimens or surgical spaying. Two injections are typically given 24 hours apart, ideally between days 0–45 after mating, with a success rate of approximately 95–98% when administered correctly. The average cost ranges from $150–$400 depending on the dog's weight and veterinary region.
Always consult your veterinarian before pursuing any pregnancy termination protocol for your dog.
The Endocrinology Behind Alizin: How Aglepristone Blocks Pregnancy
Progesterone's Role in Canine Pregnancy
In dogs, progesterone is the single most critical hormone for maintaining pregnancy. Unlike humans, where the placenta takes over progesterone production in the second trimester, dogs rely on the corpora lutea (structures on the ovaries) as the sole source of progesterone throughout the entire ~63-day gestation. This means the ovaries must remain functional for pregnancy to continue — there is no placental backup.
Progesterone performs several functions during canine gestation:
- Uterine quiescence: Suppresses myometrial contractions, preventing premature labor
- Endometrial support: Maintains the thickened uterine lining where embryos implant
- Cervical closure: Keeps the cervix tightly closed to protect developing fetuses
- Immune modulation: Prevents maternal immune rejection of fetal tissue
Serum progesterone in pregnant bitches typically remains above 2 ng/mL throughout gestation, dropping sharply only 24–48 hours before whelping. This is the vulnerability that aglepristone exploits.
Pharmacological Mechanism of Aglepristone
Aglepristone is a competitive progesterone receptor antagonist. Its molecular structure closely resembles progesterone, allowing it to bind to progesterone receptors (PR-A and PR-B isoforms) in uterine tissue with high affinity — but without activating them. Essentially, aglepristone occupies the receptor "parking spots" so that natural progesterone cannot dock and exert its effects.
The key pharmacological parameters of aglepristone in dogs:
| Parameter | Value |
|---|---|
| Receptor binding affinity | ~3× higher than progesterone for canine PR |
| Bioavailability (subcutaneous) | Rapid absorption, peak plasma within 2–4 hours |
| Half-life | Approximately 24–36 hours in dogs |
| Metabolism | Hepatic (liver), excreted via bile and feces |
| Standard dose | 10 mg/kg body weight, subcutaneous injection |
| Protocol | Two injections, 24 hours apart |
Within 48–72 hours of the second injection, the endometrium begins to detach, and embryonic or fetal tissue is resorbed (if early in gestation, before ~day 35) or expelled (if later). The corpora lutea may remain structurally present but become functionally irrelevant once their signal is blocked at the receptor level.
Evolutionary Context: Why Canine Reproduction Is Uniquely Vulnerable to Antiprogestogens
The Wolf Ancestry of Progesterone Dependence
Domestic dogs (Canis lupus familiaris) inherited their reproductive architecture from gray wolves, which evolved as monoestrous seasonal breeders. Wolves typically cycle once per year (late winter), and pregnancy must be maintained under harsh environmental conditions with limited caloric availability. This evolutionary pressure selected for a reproductive system where:
- Progesterone alone maintains pregnancy — no redundant hormonal pathways evolved because a single reliable signal was sufficient in the wolf's tightly controlled breeding season
- The corpora lutea are the sole progesterone source — unlike species with multiple estrous cycles per year (polyestrous), wolves did not need a placental progesterone handoff mechanism
- Pregnancy loss is an all-or-nothing event — in the wild, partial litter loss due to hormonal fluctuation would waste an entire breeding season, so the system evolved to be robustly binary
This evolutionary "simplicity" is precisely what makes dogs so responsive to antiprogestogen drugs. A single receptor blockade effectively terminates the entire pregnancy because there is no compensatory hormonal pathway. In contrast, species like horses or humans, where the placenta produces progesterone independently, are far less responsive to PR antagonists alone.
Practical Protocol: How Alizin Is Administered in Veterinary Practice
Alizin is a prescription-only veterinary medication and must be administered by or under the direct supervision of a licensed veterinarian. It is not available over the counter and is not approved for owner self-administration in any jurisdiction where it is marketed.
Standard Dosing Protocol
The manufacturer (Virbac) specifies the following protocol:
- First injection: Aglepristone at 10 mg/kg (0.33 mL/kg of the 30 mg/mL solution), administered subcutaneously, preferably in the scruff of the neck
- Second injection: Same dose, exactly 24 hours after the first injection
- Timing window: Can be administered from day 0 to day 45 after mating, though efficacy and resorption vs. expulsion outcomes vary by gestational stage
Outcomes by Gestational Stage
| Gestational Timing | Expected Outcome | Owner Observations |
|---|---|---|
| Days 0–25 (pre-implantation/early implantation) | Embryonic resorption — tissue is absorbed by the mother's body | Often no visible signs; mild vulvar discharge possible |
| Days 25–35 (mid-gestation) | Partial resorption, partial expulsion | Mucoid to bloody vulvar discharge over 3–7 days; mild lethargy |
| Days 35–45 (late gestation) | Fetal expulsion | Noticeable discharge, possible straining, visible fetal tissue; closer monitoring required |
Post-Treatment Monitoring
Your veterinarian will typically recommend a follow-up ultrasound 14–21 days after treatment to confirm complete pregnancy termination and check for retained fetal tissue or developing pyometra (uterine infection). Serum progesterone may also be measured; if levels remain above 1 ng/mL two weeks post-treatment, a third injection may be necessary, though this is uncommon.
Side Effects, Contraindications, and When Alizin Signals a Problem
Common Side Effects
Clinical studies and post-marketing surveillance report the following adverse effects:
- Injection site reaction (most common, ~15–20% of dogs): localized swelling, mild pain, or a transient nodule at the subcutaneous injection site, typically resolving within 7–14 days
- Vulvar discharge: Expected physiological response; ranges from clear/mucoid to bloody depending on gestational stage
- Transient lethargy or appetite reduction: Usually resolves within 48 hours
- Mammary development: Some bitches may show temporary mammary enlargement due to hormonal fluctuation; this typically regresses within 2–4 weeks
Contraindications — When Alizin Should NOT Be Used
- Known or suspected pyometra: Open or closed pyometra requires surgical intervention (ovariohysterectomy), not medical pregnancy termination
- Hepatic insufficiency: Since aglepristone is metabolized by the liver, dogs with significant liver disease may not clear the drug effectively
- Adrenal disorders (e.g., Addison's disease, Cushing's disease): Aglepristone has some affinity for glucocorticoid receptors, which could destabilize dogs with existing adrenal dysfunction
- Beyond day 45 of gestation: Late-term termination carries higher risks of hemorrhage, dystocia, and incomplete expulsion
- Heavy vaginal bleeding (soaking through bedding, continuous flow)
- Fever above 103.5°F (39.7°C)
- Foul-smelling vaginal discharge (suggests infection)
- Persistent vomiting or refusal to eat for more than 48 hours
- Signs of abdominal pain, hunched posture, or collapse
- No discharge within 5 days of treatment when administered after day 25 (may indicate treatment failure)
Common Misconceptions About Alizin and Canine Pregnancy Termination
| Myth | Fact | |
|---|---|---|
| "Alizin is a form of canine abortion pill you can buy online." | → | Alizin is a prescription-only veterinary drug that must be administered by injection. Products sold online claiming to be Alizin are often counterfeit, improperly stored, or entirely different substances. Purchasing prescription veterinary drugs from unverified sources is dangerous and illegal. |
| "It works the same way as the human abortion pill (mifepristone)." | → | While both are antiprogestogens, aglepristone is a distinct molecule with different receptor binding kinetics optimized for canine progesterone receptors. Mifepristone has been studied in dogs but is not the approved active ingredient in Alizin, and dosing protocols differ significantly. |
| "Alizin makes the dog permanently infertile." | → | Aglepristone does not destroy ovarian tissue or permanently alter the reproductive tract. Studies show that bitches treated with Alizin can conceive normally in subsequent estrous cycles. However, if the underlying reason for unwanted mating isn't addressed (e.g., intact male access), re-breeding will occur. |
| "It's safer and cheaper than just spaying the dog." | → | Not necessarily. Alizin terminates one pregnancy but leaves the dog intact, meaning she will cycle again (typically every 6–8 months) and risk future unwanted pregnancies, pyometra, and mammary tumors. A single ovariohysterectomy (spay) costs $200–$600 and permanently eliminates all these risks. Alizin is appropriate when preserving future breeding potential is the goal (e.g., a valuable breeding bitch accidentally mated to the wrong stud). |
| "You can use it at any stage of pregnancy." | → | The approved window is days 0–45 post-mating. After day 45, fetal skeletons have mineralized, and expulsion carries risks of dystocia, hemorrhage, and retained fetal membranes. Later terminations require different protocols and more intensive monitoring. |
How Is This Different from Humans?
The human and canine reproductive systems handle progesterone very differently, which is why antiprogestogens work differently across species:
| Feature | Dogs | Humans |
|---|---|---|
| Progesterone source during pregnancy | Corpora lutea only (ovarian) | Corpora lutea early, then placenta takes over (~week 8–10) |
| Response to PR antagonists | Highly effective — single pathway blockade terminates pregnancy | Effective early (before placental takeover); less reliable later without prostaglandin co-administration |
| Estrous cycle | Monoestrous/diestrous — typically 2 cycles/year | Polyestrous — monthly menstrual cycles |
| Pregnancy detection | Ultrasound from ~day 25; relaxin blood test from ~day 28 | hCG blood/urine test from ~day 10–14 post-conception |
| Gestation length | ~63 days (range 58–68) | ~280 days (40 weeks) |
This comparison illustrates why canine pregnancy is pharmacologically "simpler" to terminate with a single drug class — the dog's reproductive system never evolved the redundant progesterone pathways that make human medical abortion a multi-drug protocol.
Breed Variations and Special Considerations
While aglepristone's mechanism of action is consistent across breeds, several breed-specific factors influence clinical decision-making:
- Brachycephalic breeds (English Bulldogs, French Bulldogs, Pugs): These breeds already have high rates of dystocia and often require cesarean sections. If pregnancy termination is desired, Alizin is generally preferred over allowing a high-risk pregnancy to proceed, but the injection site reaction may be more pronounced in dogs with thick, folded neck skin. Your vet may choose an alternative injection site.
- Giant breeds (Great Danes, Irish Wolfhounds): The cost of Alizin scales with body weight (10 mg/kg), so a 70 kg Great Dane requires 700 mg per injection — significantly more expensive than treating a 5 kg Chihuahua. Total treatment cost for giant breeds may reach $400–$600.
- Sighthounds (Greyhounds, Whippets): These breeds have lower body fat percentages, which can affect subcutaneous drug absorption kinetics. Your veterinarian may monitor more closely or adjust injection technique.
- Breeding bitches in competitive programs: For dogs in active breeding programs (show lines, working dog programs), Alizin offers the advantage of preserving reproductive potential. The bitch can be re-bred at her next cycle without the permanent fertility loss associated with spaying.
Alternatives to Alizin: Other Approaches to Unwanted Canine Pregnancy
For owners in countries where Alizin is unavailable (including the United States) or for cases where aglepristone is contraindicated, several alternatives exist:
| Option | Mechanism | Efficacy | Cost Range | Key Limitation |
|---|---|---|---|---|
| Ovariohysterectomy (spay) | Surgical removal of ovaries and uterus | 100% | $200–$600 | Permanent; eliminates future breeding potential |
| Prostaglandin F2α (e.g., cloprostenol) | Luteolysis — destroys corpora lutea, dropping progesterone | 85–95% (protocol-dependent) | $100–$300 | Multiple injections over several days; higher side effect profile (vomiting, panting, diarrhea) |
| Cabergoline + cloprostenol combination | Cabergoline suppresses prolactin; cloprostenol induces luteolysis | 90–97% | $200–$500 | Requires daily oral cabergoline for 5–10 days plus injections |
| "Mismatch" injection (estrogen-based) | Prevents embryo implantation via estrogen excess | Variable; ~70–85% | $50–$150 | Not recommended — significantly increases pyometra risk; considered outdated by most veterinary theriogenologists |
Frequently Asked Questions About Alizin for Dogs
Can I buy Alizin online and give it to my dog myself?
No. Alizin is a prescription-only veterinary medication in every country where it is approved. It must be administered by a licensed veterinarian via subcutaneous injection. Purchasing aglepristone from online pharmacies without a prescription is illegal in most jurisdictions and dangerous — counterfeit products, improper storage (aglepristone requires temperature-controlled conditions), and incorrect dosing can lead to treatment failure, infection, or serious adverse effects. If you need pregnancy termination for your dog, contact your veterinarian directly.
Is Alizin the same as the "morning-after pill" for dogs?
Not exactly. While the concept is similar — preventing or terminating an unwanted pregnancy after mating — Alizin is not a single-dose oral medication. It requires two subcutaneous injections 24 hours apart and can be administered up to 45 days after mating, not just within a narrow post-coital window. There is currently no approved oral "morning-after pill" for dogs in any major market.
Will my dog be in pain during or after Alizin treatment?
Most dogs tolerate the subcutaneous injections well, similar to a routine vaccination. The injection site may be sore for a few days. After treatment, dogs undergoing fetal expulsion (typically when treated after day 35) may show mild cramping and discomfort, similar to menstrual cramps. Your veterinarian may prescribe a short course of an NSAID (such as meloxicam or carprofen) for pain management. Serious pain is uncommon and should prompt a veterinary recheck.
How soon after Alizin treatment can my dog be bred again?
Dogs can return to estrus as early as 2–4 months after Alizin treatment, depending on their individual cycle timing. There is no mandatory waiting period imposed by the drug itself. However, responsible breeders typically wait at least one full cycle to allow the endometrium to fully recover and to confirm normal cyclicity via progesterone monitoring before attempting another breeding.
Is Alizin available in the United States?
No. As of 2026, aglepristone (Alizin) is not FDA-approved for veterinary use in the United States. U.S. veterinarians who need to terminate canine pregnancies typically use prostaglandin-based protocols (e.g., cloprostenol injections), sometimes combined with cabergoline, or recommend ovariohysterectomy. Some U.S. veterinary theriogenologists may have access to aglepristone through special import or compounding channels, but this is uncommon. Discuss available options with your veterinarian.
What does it mean when my dog has discharge after Alizin?
Vulvar discharge after Alizin treatment is a normal and expected sign that the medication is working. The discharge may be clear, mucoid, or bloody, depending on the gestational stage at treatment. It typically lasts 3–10 days. However, if the discharge becomes foul-smelling, is accompanied by fever, or if your dog becomes lethargic, contact your veterinarian immediately — these may indicate a secondary uterine infection (pyometra).
Can dogs get pregnant while on Alizin?
Alizin terminates an existing pregnancy; it does not provide ongoing contraception. Once the drug is metabolized (within approximately one week), the dog is fully capable of conceiving again if she is mated during her next estrous cycle. If you do not intend to breed your dog, discuss permanent contraception (spaying) with your veterinarian.
References
- Fieni, F. (2006). "Clinical evaluation of the use of aglepristone for pregnancy termination in the bitch." Theriogenology, 66(3-4), 560–564. — Key clinical trial establishing the 95–98% efficacy rate of the two-injection protocol.
- Galac, S., et al. (2000). "Termination of mid-gestation pregnancy in bitches with aglepristone." Theriogenology, 53(4), 941–950. — Demonstrated efficacy of aglepristone at later gestational stages.
- Merck Veterinary Manual — Pregnancy and Parturition in Small Animals: Comprehensive reference on canine reproductive physiology and pharmacological interventions.
- FDA Center for Veterinary Medicine: Current database of approved animal drugs in the United States.
Beth Carrasco
All our authors care for dogs every day — read more of their work on the authors page.
