Estrogen Side Effects in Dogs

The side effects of using estrogen, often prescribed to treat urinary incontinence in female dogs, depend on the type of hormone replacement therapy for dogs your veterinarian prescribes. Generally, urinary incontinence in spayed female dogs occurs because of decreased tone in the urethral sphincter, which estrogen alleviates.

Many of estrogen's side effects in spayed dogs are transient or mild. Spaying removes at least the ovaries and often the uterus as well. But some kinds of estrogen can cause toxicity such as bone marrow suppression in large doses. This is a very serious condition that can be fatal.

‌All estrogen medications are intended for spayed female dogs unless specifically given to abort a pregnancy.‌ However, estrogen is not the veterinary drug of choice to induce canine abortion. Pregnant or nursing canines should not receive any of these estrogen formulations, nor should male dogs.

Furthermore, dogs with mammary cancer or tumors should not receive these drugs, nor should a dog suffering from bone marrow issues. Tell your veterinarian about any other medications or supplements your dog receives.

Estradiol cypionate (ECP) is a semisynthetic hormone. It's important to note that this drug is not likely to be prescribed to a dog with urinary incontinence thanks to the development of Incurin, the brand name of a drug that contains estriol, a natural estrogen for dogs.

At high doses, estradiol cypionate can cause gastrointestinal issues. Symptoms of gastrointestinal issues include:

• Vomiting
• Diarrhea
• Appetite loss

Other side effects of estrogen, specifically estradiol, include:

• Excessive drinking
• Excessive urination
• Lethargy
• Alopecia

If your dog experiences unusual bruising or bleeding, or any other abnormalities, stop giving them the medication and call your veterinarian. Clinical signs, like bruising or bleeding along with anemia, can be indicators of bone marrow suppression -- an estradiol side effect -- or thrombocytopenia, low blood platelets. Bone marrow suppression can lead to death.

Female dogs who aren't spayed also have a high risk of developing pyometra (a uterine infection) after receiving estradiol cypionate. Furthermore, this drug was once used to induce abortion in dogs (called a mismating shot), but the drug is no longer recommended for that purpose.

A nonsteroidal estrogen, diethylstilbestrol, or DES, resembles natural estrogen. It's no longer available at commercial pharmacies, but a compounding pharmacy can fill your veterinarian's order. It was removed from the market because studies indicated that it can cause cancer in humans, though it does not appear to have that effect in canines and is still used in veterinary medicine.

Currently, DES is used only for treating canine incontinence. At recommended doses, DES is generally considered safe.

Check with your veterinarian but the recommended dosage is usually one pill per day at first. When the incontinence appears under control, stop the DES administration and observe when your dog starts dribbling urine again. If they dribble after four days, you'll give them DES every fourth day. If they do it again only after five days, you'll administer a pill every fifth day, and so on.

At high concentrations or doses, DES can cause the following side effects:

• Bone marrow suppression (which is very serious)
• Hair loss
• Vomiting
• Diarrhea
• Appetite loss
• Lethargy

In 2012, Merck Animal Health introduced an FDA-approved medication for female canine urinary incontinence called Incurin‌.‌ Incurin tablets contain estriol, a natural estrogen of short duration in the dog's body.

Because estriol binds with estrogen receptors in the body for only one to four hours —much less than other estrogen-based canine urinary drugs — it doesn't cause potential side effects like bone marrow suppression. However, while Incurin is generally well tolerated, some dogs do experience side effects while taking it.

Incurin side effects include the following:

• Swollen vulva
• Excessive drinking and urination
• Appetite loss
• Vomiting

Tell your veterinarian if your dog exhibits any of these side effects while on Incurin.

Urinary incontinence in spayed female dogs is an estrogen-responsive condition. There are several types of estrogen for dogs, including estradiol cypionate, estriol, and diethylstilbestrol, but Incurin, the brand name of estriol used in dogs, is commonly prescribed to reduce the incidences of canine incontinence because it has the fewest adverse effects and contains a natural rather than synthetic estrogen. Phenylpropanolamine is an estrogen alternative for treating urinary incontinence.