Overview of Estradiol for Canines and Felines

Estradiol cypionate, better known as Estrace® or ECP®, is a semisynthetic estrogenic hormone used as a replacement therapy in conditions of inadequate estrogen production for dogs and cats.

It is also used by injection to prevent embryo implantation in dogs and cats if given within 72 hours of mating.

Estradiol is the most active endogenous estrogen. It is responsible for the development of secondary sexual characteristics; increases calcium deposition in bones and accelerates epiphyseal closure; has a slight anabolic effect; and increases retention of sodium and water.

Exogenous estradiol is distributed throughout the body, accumulating in fat.

In most species, it is metabolized by the liver and the metabolites are excreted in both urine and bile. In cats, estrogen metabolites are excreted preferentially in feces (in equal amounts of unconjugated estradiol and conjugates).

This drug is not approved for use in dogs and cats by the Food and Drug Administration but it is prescribed legally by veterinarians as an extra-label drug.

Estradiol is a prescription drug and can only be obtained from a veterinarian or by prescription from a veterinarian.

Brand Names and Other Names of Estradiol

This drug is registered for use in humans and cattle.

Human formulations: Estrace® (Warner Chilcott), Gynodiol® (Fielding), and generic estradiol.

Veterinary formulations: ECP® (Pharmacia) [approved for used in cattle] and generic estradiol cypionate.

Uses of Estradiol for Dogs and Cats

The uses of estradiol include:

To prevent implantation of the fertilized ova in mismatched bitches.

Palliative treatment for benign anal tumors in elderly male dogs.

Hormone replacement therapy for spayed female dogs, particularly those with urinary incontinence (although another drug called DES is used more commonly).

Treatment of anestrus.

Precautions and Side Effects

Estradiol should not be used in animals that appear hypersensitive to it or any components of the medication.

Use of estradiol to prevent embryo implantation is subsequently associated with a high incidence of cystic endometrial hyperplasia and pyometra.

Signs of estrus may also occur.

In males, estradiol can cause feminization.

Estrogens are considered toxic to bone marrow in small animals and may lead to blood dyscrasias.

Overdose of estrogen has been reported to cause anemia and thrombocytopenia.

Drug Interactions

Estradiol may interact with other medications. Consult with your veterinarian to determine if other drugs your pet is receiving could interact with estradiol.

Estradiol enhances absorptive functions of the dog proximal small intestine and can affect pharmacokinetics of some orally administered drugs.

Estrogens in general may reduce the effects of anticoagulants.

They may also alter the pharmacological effect of tricyclic antidepressants, increasing their toxicity.

Estrogens also increase the pharmacologic and toxologic effects of corticosteroids and increase thyroid hormone requirements in hypothyroid patients.

Cytochrome inducers (e.g. barbiturates) decrease plasma levels and thus the effectiveness of estrogens.

Cytochrome inhibitors (e.g. ketoconazole and macrolide antibiotics) decrease the plasma levels and effectiveness of estrogens.

Loss of seizure control may occur when estrogens are administered to hydantoin-treated epileptics.

How Estradiol is Supplied

Estradiol is available in 0.5 mg, 1 mg, 1.5 mg, or 2 mg micronized estradiol tablets.

The injectable forms of estradiol cypionate include 2 and 4 mg/mL concentrations.

Dosing Information of Estradiol for Dogs and Cats

Medication should never be administered without first consulting your veterinarian.

Doses of estradiol vary widely depending on the reason for prescribing.

The duration of administration depends on the condition being treated, response to the medication and the development of any adverse effects. Be certain to complete the prescription unless specifically directed by your veterinarian. Even if your pet feels better, the entire treatment plan should be completed to prevent relapse.

In dogs, the usual dose for mismating is 0.01 to 0.02 mg per pound (0.02 – 0.04 mg/kg) given intramuscularly.

In cats, the usual dose for mismating is 0.0625 to 0.125 mg per pound (0.125-0.25 mg/kg) intramuscularly.

Estradiol is uncommonly recommended by most veterinarians for use for mismating due to possible side effects and safer alternative drugs.