The side effects of estrogen, often prescribed to treat urinary incontinence in female dogs, depend on the type of estrogen your vet 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, but some kinds of estrogen can cause bone marrow suppression in large doses.
Estradiol Cypionate in Dogs
Estradiol cypionate is a semisynthetic hormone. At high doses, estradiol can cause gastrointestinal issues. Symptoms include vomiting, diarrhea and appetite loss. Other side effects include excessive drinking and urination, and lethargy. If your dog experiences unusual bruising or bleeding, stop giving her the medication and call your veterinarian. That bruising or bleeding, along with anemia, can be a sign of bone marrow suppression -- an estradiol side effect -- or thrombocytopenia, low blood platelets. Bone marrow suppression can lead to death.
Diethylstilbestrol in Dogs
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 it caused cancer in people -- it does not appear to have that effect in canines.
Currently, DES is used only for treating canine incontinence. At recommended doses, DES is quite safe and has no side effects.
Check with your vet, but recommended dosage is usually one pill per day, at first. When the incontinence appears under control, you'll stop the DES administration and observe when your dog starts dribbling urine again. If she does it after four days, you'll give her DES every fourth day; if she dribbles only after five days, you'll administer a pill every fifth day, and so on. At high doses, DES causes bone marrow suppression, along with less serious side effects such as hair loss.
Estriol in Dogs
In 2012, Merck Animal Health introduced a 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 such as bone marrow suppression.
However, while Incurin is generally well-tolerated, some dogs do experience side effects while taking it. These include vulvar swelling, excessive drinking and urination, appetite loss and vomiting. Tell your vet if your dog exhibits any of these side effects while on Incurin.
Precautions and Contraindications
All estrogen medications are indicated for spayed female dogs, unless given specifically to abort a pregnancy. However, estrogen is not the veterinary drug of choice to induce canine abortion. Pregnant or nursing dogs should not receive any of these estrogen formulations, nor should male canines. Dogs with mammary cancer should not receive these drugs, nor should a dog suffering from bone marrow issues. Tell your vet about any other medications or supplements your dog receives.