What is Anti-estrogen Therapy?

Anti-estrogen therapy is used to treat cancers that manifest in hormone-sensitive tissues. Cancer of the reproductive tissues in dogs, such as uterine cancer or breast cancer, are usually treated with therapy focusing on estrogen. Hormone therapy works by either decreasing or increasing the levels of hormones in the canine body. Due to the fact that hormones stimulate cellular growth, it makes sense that altering the levels of hormones can affect how a cancerous tumor grows. Estrogen, specifically, promotes the growth of some forms of breast cancer, so decreasing the level of estrogen in the body would ideally slow the growth of the tumor. Anti-estrogen therapy is prescribed by a veterinary specialist, but the course of therapy can be administered at home. 

Anti-estrogen Therapy Procedure in Dogs

Anti-estrogen therapy (anti-hormone therapy) medications are usually in pill form, given as directed by your veterinary specialist. Hormone therapy can be given in the comfort of the canine’s home, but may be given in a hospital setting if the dog is undergoing other forms of cancer treatment. Anti-estrogen therapy is a combination of selective estrogen receptor modulators and diethylstilbestrol (DES) drugs.

Selective estrogen receptor modulators act like estrogen on some parts of the body, but block the effects on other tissues like the mammary glands. These drugs are both estrogenic and anti-estrogenic, therefore, the SERMs are able to effect each tissue in a different way. Your veterinarian may prescribe selective estrogen receptor modulators (SERMs) such as; Tamoxifen, Toremifene, or Raloxifene.

Diethylstilbestrol (DES) is a synthetic estrogen primarily used for breast cancer. Diethylstilbestrol often replaces tamoxifen to provide a less toxic option for pet owners, but has been associated with blood clots. 

Efficacy of Anti-estrogen Therapy in Dogs

The efficacy of anti-estrogen therapy is guarded to good in canines. The use of hormone replacement therapy is highly effective in humans, but the overall effects of replacing hormones is still undergoing studies in the world of veterinary medicine. This form of cancer treatment is often used after other forms of treatment have proven ineffective.

Anti-estrogen Therapy Recovery in Dogs

Anti-estrogen therapy (anti-hormone therapy) are part of a canine’s treatment plan that may last weeks, months, or years depending on the veterinarian’s recommendations. Dog owners should expect routine follow-up appointments with the veterinarian during the therapy process. The therapy regimen may be altered as the hormones begin circulating in the body and the effects of these chemicals begin to take place. Some side effects are to be expected during the therapy process, but it is important to monitor the canine during treatment to detect severe side effects early. 

Cost of Anti-estrogen Therapy in Dogs

Anti-estrogen therapy involves a combination of selective estrogen receptor modulators and diethylstilbestrol (DES) drugs. Tamoxifen costs about $100 a month (30 day supply), Toremifene can cost over $1,000 for a month’s supply and Raloxifene costs about $170 for a month’s supply. 

Dog Anti-estrogen Therapy Considerations

Like all forms of hormone therapy, anti-estrogen therapy does have its side effects but most are not usually life-threatening. The more common side effect related to anti-estrogen is gastrointestinal upset, manifested by vomiting and nausea. Other side effects include uterine bleeding, high levels of blood calcium, fluid retention, and a decreased interest in mating. Female dogs often experience a change in behavior, vaginal bleeding, and discharge. There has also been reports of dogs developing cataracts, blood clots, and uterine cancer, but these conditions are quite rare. 

Anti-estrogen Therapy Prevention in Dogs

Anti-estrogen therapy is used as a cancer treatment for hormone-sensitive tissues. Like all forms of cancer, preventing the rapid growth of cells resulting in a growth is unknown. Veterinary health professionals recommend a healthy lifestyle as a method of prevention, but even with a proper diet and exercise, cancer can still affect your dog. Until further research is completed, there is no known prevention method for any form of cancer. 

Anti-estrogen Therapy Questions and Advice from Veterinary Professionals

Yorkshire Terrier
6 Years
Serious condition
0 found helpful
Serious condition

Has Symptoms


I have a 6 pound yorkie female with a mamory gland lump the size of a golf ball and 5 other smaller ones by her niples. My vet wants to do surgary but I don't want to risk the chance of her not being able to nurse puppies. I want to breed her but won't do it with the lumps. I have tried several remodies found on the internet supposed to cure cancer in dogs. They were a waste of my money and did not do a thing for my dog she still has the lumps. I want something that really works.
I took femara myself years ago for cancer that was estrogen receptive. Is there a hormone blocker ( letrozole) that I can give my dog to get rid of the lumps and if so what is it called and what is the dosage. Thanks Lily

Dr. Callum Turner, DVM
Dr. Callum Turner, DVM
3320 Recommendations
If it was easy to cure cancer with stuff bought of the internet, many people would still be alive today and I’m sure pharmaceutical companies would pounce on them to make them their own. Unfortunately I agree with your Veterinarian that surgery is the treatment of choice to remove the lumps and to send them for histopathology, I also would recommend against breeding with her and would spay her at the same time as removing the lumps; I know this is the exact opposite of what you want to hear. Hormonal therapy for mammary lumps isn’t something done really in veterinary medicine due to the ease of surgery and ovariohysterectomy. Regards Dr Callum Turner DVM

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Labrador Retriever
12 Years
Moderate condition
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Moderate condition

Has Symptoms

Vaginal Discharge

I'm very certain my dog is producing too much estrogen.she had tests dobe last year and showed no sign of cancer just a uti which was taken care of with medications. I have an appointment on Saturday morning with the vet. Should I suggest what i think is going on?? She has loss of appetite hair loss and white vaginal doscharge.

Dr. Michele King, DVM
Dr. Michele King, DVM
1611 Recommendations
Thank you for your email. it is very uncommon for a dog to produce too much estrogen. If she isn't spayed, she is at risk for uterine infections and uterine cancer. Without more information about her, I can't comment on what might be going on, but it would be a good idea to point out the things that you have noticed to your veterinarian, as that may help them decide what is going on and recommend any tests or treatments for her. I hope that she does well.

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