Aromatase Inhibitors

What are Aromatase Inhibitors and how do they work?

Aromatase inhibitors are drugs that prevent an enzyme, aromatase, from converting androgens (male sex hormones) into estrogen (female sex hormone). This directly reduces the amount of estrogen in the body as opposed to blocking the estrogen receptor like the other kinds of hormonal therapy drugs. There are two types of aromatase inhibitors approved for treatment of breast cancer.[1] These include:

  • Irreversible steroidal inhibitors
  • Non-steroidal inhibitors

When are Aromatase Inhibitors Indicated?

Aromatase inhibitors are only effective if a patient has hormone receptor-positive breast cancer. They are typically used in postmenopausal women whose estrogen is being produced by the adrenal glands as opposed to the ovaries. They are not indicated for premenopausal or hormone receptor-negative patients. Aromatase inhibitors are typically given as adjuvant therapy, meaning it is used after surgery or with another form of treatment if no surgery is performed.[2]

Benefits of Aromatase Inhibitors

Aromatase inhibitors are designed to prevent cancer recurrence in postmenopausal, hormone-receptor-positive breast cancer. In fact, it has been shown to have synergistic effects with other types of hormonal therapy. Research has shown the following:

  • For those who take tamoxifen for 2-3 years followed by 2 years of aromatase inhibitors, survival is superior to the typical tamoxifen regimen, which is tamoxifen alone for 5 years.
  • Furthermore, if you take tamoxifen for 5 years and aromatase inhibitors for another 5 years, your risk of recurrence is reduced substantially.

As great as this sounds, all drugs have side effects and may not be tolerated for the length of time required in some patients.

Side Effects and Risks of Aromatase Inhibitors

First generation aromatase inhibitors were broken down in the body to form aminoglutethamide, which caused several serious side effects. While the current generation of aromatase inhibitors has less harmful side effects, there are still a few to be concerned about.

These may include:

  • Joint pain or stiffness
  • Decreased bone density – leading to osteoporosis
  • Hot flashes
  • Perspiration or increased sweating
  • Headaches
  • Chest pain
  • Elevated cholesterol
  • Heart attack
  • Nausea/vomiting
  • Diarrhea
  • Blood clots
  • Back pain
  • Hair loss

Some of these side effects may be minor and tolerable for most patients, however, some may not. The more serious side effects such as heart attack, blood clots and osteoporosis may cause your physician to stop giving you this medicine. Some may just encourage you to take calcium and vitamin D supplements to improve your bone density.


Aromatase Inhibitors are a great adjunct to other forms of treatment in hormone receptor-positive patients. Your doctor will discuss whether or not they are indicated in your particular cancer and as a team, you can decide what the best treatment regimen is for you.


1 Lønning, PE (2011). The Potency and Clinical Efficacy of Aromatase Inhibitors across the Breast Cancer Continuum. Annals of Oncology 22: 503–514.

2 Winer, EP; Hudis, C; Burstein, HJ; Wolff, AC; Pritchard, KI; Ingle, JN; Chlebowski, RT; Gelber, R; Edge, SB; Gralow, J; Cobleigh, MA; Mamounas, EP; Goldstein, LJ; Whelan, TJ; Powles, TJ; Bryant, J; Perkins, C; Perotti, J; Braun, S; Langer, AS; Browman, GP; Somerfield, MR (2005). American Society of Clinical Oncology Technology Assessment on the Use of Aromatase Inhibitors as Adjuvant Therapy for Postmenopausal Women with Hormone Receptor–Positive Breast Cancer: Status Report 2004. Journal of Clinical Oncology, 23(3):619-629.

This article was originally published on July 27,2012 and last revision and update of it was 9/2/2015.