A review of Tamoxifen VS Anastrozole


A review of Tamoxifen VS Anastrozole

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Breast cancer cannot be discussed without the mention of anastrozole or tamoxifen. The estrogen receptor-positive breast cancer makes up 75–80% of all breast cancer cases worldwide. 

Anastrozole and tamoxifen are effective in treating breast cancer. The choice between either of them depends on many factors, because of their different effects on estrogen. These will be explained below.


Breast cancer is one of the most common diseases worldwide. Despite early intervention, its incidence is still high. Estrogen receptor-positive breast cancer is the only subtype that is increasing because it makes up 75–80% of all breast cancer cases.

Estrogen is essential in the development of the female sex organs and the regulation of the menstrual cycle and reproduction. Therefore, in experimental studies in animals, it has been proposed that estrogen may be involved in some cases of breast cancer. There is an increase in the risk of breast cancer with advancing age due to the loss of ovarian function in either menopause or bilateral oophorectomy.

In premenopausal rats, the predominant form of circulating estrogen is estradiol secreted by the ovaries monthly. However, after menopause, the production of estrogens in the ovaries ceases, and the source of estradiol is by conversion from estrone, itself produced mostly through the peripheral conversion of androgen precursors, predominantly androstenedione, in extraglandular tissues, such as adipose tissue, breast, heart, brain, etc.


Anastrozole is a third-generation non-steroidal aromatase inhibitor for the aromatase enzyme. It binds to it reversibly than other generations of AIs. It is a member of the nitrile and triazole family. Treatment with anastrozole suppresses plasma levels of estrone, estradiol, and estrone sulfate, irrespective of the dose in animal test subjects.

Anastrozole may play a vital role as an antineoplastic agent. According to the data obtained through clinical studies on rats, anastrozole treated breast cancer cells by shrinking them.


Anastrozole interferes with the body’s ability to produce estrogen from androgens by suppressing aromatase enzyme activity, blocking estrogen synthesis.

The ovary is responsible for estrogen circulation in premenopausal females. Following menopause, estrogen is produced outside the ovaries in tissues such as the breast, uterus, vagina, brain, bone, heart, etc.


Tamoxifen is a nonsteroidal selective estrogen receptor modulator (SERM). It exhibits both estrogenic agonist and antagonist effects in different parts of the body of animal test subjects. In the breast tissue, it is an antagonist, causing antiestrogenic and antitumor effects. Through downstream intracellular processes, it slows cell cycling, which classifies it as cytostatic. In the bone, it stimulates estrogen receptors instead of blocking them, exerting an estrogenic agonist effect, and may prevent osteoporosis.


It competitively inhibits the binding of estradiol to estrogen receptors, thereby preventing the receptor from binding to the estrogen-response element on DNA. This results in a reduction in DNA synthesis and cellular response to estrogen.


In premenopausal states, anastrozole has limits in reducing circulating estrogen. Therefore, it is not given to premenopausal females for breast cancer suppression without adding medication to decrease hormone levels.

Tamoxifen, on the other hand, is useful for treating advanced disease in postmenopausal females. However, it may benefit premenopausal females with estrogen receptor-positive disease.


The decision to use anastrozole as initial endocrine therapy, as opposed to switching after 2–3 years of tamoxifen therapy, is guided by the tumor’s characteristics. Females who have ER-positive tumors with unfavorable characteristics, such as HER-2 positivity, PgR negativity or nodal positivity, are likely to be selected for immediate anastrozole therapy. However, females with ER-positive tumors without unfavorable characteristics, are likely selected for tamoxifen treatment for 2–3 years before taking anastrozole for 2–3 years.


You can purchase Anastrozole and Tamoxifen from Loti Labs. Buy research liquids that are USA-made for the integrity of your research.


  1. Fabian C. J. (2007). The what, why, and how of aromatase inhibitors: hormonal agents for treatment and prevention of breast cancer. International journal of clinical practice, 61(12), 2051–2063. doi:10.1111/j.1742-1241.2007.01587.x
  1. Forbes, J. F., Sestak, I., Howell, A., Bonanni, B., Bundred, N., Levy, C., … IBIS-II investigators (2016). Anastrozole versus tamoxifen for the prevention of locoregional and contralateral breast cancer in postmenopausal women with locally excised ductal carcinoma in situ (IBIS-II DCIS): a double-blind, randomized controlled trial. Lancet (London, England), 387(10021), 866–873. doi:10.1016/S0140-6736(15)01129-0
  1. ESHRE Capri Workshop Group, Hormones, and breast cancer, Human Reproduction Update, Volume 10, Issue 4, July 2004, Pages 281–293, https://doi.org/10.1093/humupd/dmh025

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