Anti-Hormonal Therapy and Breast Cancer

Anti-hormonal therapy is used to treat estrogen receptor (ER) and/or progesterone receptor (PR) positive breast cancer. Most breast cancers are hormone receptor positive. This means that the cancer was fed or grown by the female hormones.  

Note: If your cancer was negative for estrogen receptors (ER) and progesterone receptors (PR) then your cancer was not fed by these hormones and the anti-hormonal therapy would not work for you.

When women are premenopausal (having periods), the ovaries make the hormones estrogen and progesterone. Women are considered postmenopausal when they have not had a period for over one year. A blood test can confirm menopause and this means the ovaries do not make the female hormones anymore. Menopause can happen naturally with age, sometimes after chemotherapy or immediately after you have had your ovaries surgically removed.

Your body makes an enzyme called aromatase. Aromatase changes the male hormones that you have (called androgens) into the female hormone estrogen. This is how estrogen is made in postmenopausal women. Estrogen is made in many tissues in your body such as the adrenal glands, fat, muscle and breast tissues. Post-menopausal women make enough estrogen to feed breast cancer.

Anti-hormonal therapy is not chemotherapy. It is usually a pill taken daily that is used to slow or stop breast cancer from growing. This treatment can reduce the risk of:

  • the cancer from coming back in the area where the cancer was
  • a new breast cancer in the other breast
  • the breast cancer spreading to other areas of the body.

Men with male breast cancer usually have hormone positive breast cancer too and can benefit from anti-hormonal therapy. 

These medications are recommended for a minimum of five years to greatly reduce the risk of your breast cancer from coming back. It may be recommended to use anti-hormonal therapy for up to 10 years. Your oncologist will discuss with you how long you will need to take it.

Examples of anti-hormonal therapy include:

  • Tamoxifen - blocks estrogen from attaching to your breast cells. It can be used by premenopausal or postmenopausal women since it does not matter where the hormones come from. Men with breast cancer would use Tamoxifen.
  •  Anastrozole, Letrozole or Exemestane: stops the aromatase enzyme from changing the androgens into estrogens.  Since this is how postmenopausal women make their estrogen, aromatase inhibitors can only be used by postmenopausal women.  
  • There other types of anti-hormonal therapy used for metastatic breast cancer (stage 4)

I’m taking anti-hormonal therapy. What do I need to know?

When you are taking Tamoxifen:

Tamoxifen is generally well tolerated. Side effects may include hot flashes, vaginal discharge or dryness and some may find their mood a little low. 

There is a small risk of developing uterine (womb) cancer when using Tamoxifen, in particular for postmenopausal women who still have a uterus. Having an annual gynecological exam is advised.   

All women on Tamoxifen should see their health care provider promptly if they have any vaginal spotting or bleeding, abnormal discharge, pelvic pain or other pelvic symptoms. In this instance, it would be recommended to have a more detailed gynecological assessment including a pelvic ultrasound, possible uterine biopsy and a referral to a gynecologist should be considered.

Tamoxifen can increase your risk of developing blood clots (deep vein thrombosis) by 2-3% and a slight increased risk for stroke. Watch for pain, redness, or swelling in your legs or shortness of breath and seek medical attention immediately. When travelling, change your position and walk around frequently. Avoid crossing your legs when sitting. Make sure you stay hydrated.

Tamoxifen is associated with a small increased risk of early cataract formation. Manitoba Health covers the cost of basic eye exams every one to two years. The eye doctor can bill Manitoba Health directly or if you pay, you can submit the bill for a re-imbursement.

**Please note that it is not safe to become pregnant while taking Tamoxifen.** If your period stopped as a result of chemotherapy, you are still at risk for getting pregnant. Do not use any type of birth control that contains any hormones. Discuss non-hormonal methods of birth control with your health care provider. Example: condoms and spermicidal jelly.

When you are taking Anastrozole, Letrozole or Exemestane (these are called Aromatase Inhibitors):

Aromatase Inhibitors are generally well tolerated. Side effects may include hot flashes, joint pain and stiffness, vaginal dryness and rarely hair thinning, nausea, diarrhea, headaches, fluid retention, low energy and rash.

Aromatase inhibitors have an increased risk of osteopenia (thinning of the bones), osteoporosis (thin bones) and fractures (breaking of bones). Have a baseline Bone Mineral Density Scan (DEXA). The bone density scan should be repeated every one to two years while taking an Aromatase Inhibitor.  If there are signs of bone loss you should have treatment with a bisphosphonate (bone building medication). Talk with your health care provider about the treatment options available.

It is recommended to take Calcium and Vitamin D supplements. Be sure to get 1200mg of calcium and 800-2000 IU of Vitamin D per day. Aim to get the majority of calcium through your diet and supplement as needed.

These medications may increase your cholesterol and blood pressure. Have your cholesterol and blood pressure monitored by your health care provider and treat if elevated.


  • Some medications are better tolerated more than others. Most medications will often have several brands available. This means that trying a different brand of your medication (made by a different company) may make a difference with side effects.
  • Time of day: Some find that if the anti-hormonal medication is taken before bed, they may sleep through the symptoms. Others find that they wake up a lot through the night, maybe due to hot flashes, and they feel very tired the next day. If this continues for a long period of time, many people find they have increased fatigue, more aches and pains, brain fog, low mood etc. Try gradually switching the time of day that you take the medication. Once you find the time of day that works best, the medication should be taken at the same time each day.

Do you need more information or help with side effects?

It is important to know that most people tolerate anti-hormonal therapy quite well. Side effects can interfere with your quality of life. These medications are an important part of your treatment plan for your breast cancer. If you experience side effects it is important to talk to someone on your health care team about them.

The Breast Cancer Patient & Family Educator at the Breast & Gyne Cancer Centre of Hope can help you understand about your cancer and treatments including
anti-hormonal therapy. As a nurse, the educator can help you with side effects and make suggestions that may help you. Call 204-788-8014 or Toll-Free 1-888-660-4866.