Arimidex is an aromatase inhibitor used to lower circulating estrogen. It was developed to help fight breast cancer as estrogen plays a role in the growth of cancer cells. Arimidex binds reversibly to the aromatase enzyme through competitive inhibition. This suppresses the conversion of androgens into estrogen. Circulating plasma estrogen can be reduced by nearly 85% in women using Arimidex. A common misconception is that aromatase inhibition is similar in men than women.

However in trials when males were administered 1mg of Arimidex daily, circulating estrogen was only reduced by about 50%. Anastrozole is rapidly absorbed orally (time to reach maximum concentration, 1 h) with a slow apparent clearance of 1.54 liters/h and a terminal half-life of 46.8 h. Because Arimidex reversibly binds to the aromatase enzyme, once you stop taking it the aromatase enzyme is free to convert androgens such as testosterone into estrogen again. This is sometimes referred to as estrogen rebound. Other aromatase inhibitors like Aromasin are irreversible and therefore are less likely to cause estrogen rebound.

arimidex chart

Figure 1. Changes in testosterone and E2 concentrations in normal young men (15–22 yr old) before () and after 10 days of oral anastrozole at 0.5 and 1 mg.

Arimidex not only lowers circulating estrogen but it also increases LH and FSH concentrations in addition to increasing testosterone by about 58% in men. In one study elderly men with mild hypogonadism were administered 1mg daily of Arimidex for 12 weeks. This treatment normalized serum testosterone levels in those men without adversely affecting lipids, inflammatory markers of cardiovascular risk or insulin resistance.

Arimidex can be employed during a steroid cycle when aromatizing compounds such as testosterone are administered in order to control estrogen from getting out of control. During the course of a typical steroid cycle estrogen can rise quite high. Estrogen has been measured as much as 7 times higher than normal in men on steroids. This is excessive and can potentially cause water retention, gynecomastia (the formation of female breast tissue) or benign prostatic hyperplasia. Therefore in order to avoid these side effects estrogen must be controlled.

Reduction in breast area and breast volume have been observed in young men treated for 6 months with Arimidex (1 mg daily). These subjects had recent preexisting gynecomastia (less than one year). However boys with longstanding gynecomastia (more than one year) were unresponsive to 6 months of Arimidex treatment, possibly due to development of dense breast fibrosis. Therefore using Arimidex to treat recent gynecomastia is supported by the data.

Arimidex may be used during a steroid cycle with aromatizing compounds and during PCT to help keep the estrogen to testosterone balance in favor of testosterone. However because Arimidex is a reversible aromatase inhibitor it may not be the best AI for PCT. From the data I have read and my years of experience with this medication, 0.5mg of Arimidex every other day is a good starting point on moderate doses of testosterone. If testosterone doses are raised then 0.5mg to 1mg daily may be needed to control estrogen. Since either high and low estrogen can cause side effects such as low libido only labs can determine the appropriate dose of Arimidex.

1. Effect of aromatase inhibition on lipids and inflammatory markers of cardiovascular disease in elderly men with low testosterone levels.
2. Estrogen suppression in males: metabolic effects.
3. Pharmacokinetics and pharmacodynamics of anastrozole in pubertal boys with recent-onset gynecomastia.
4. Influence of Neoadjuvant Anastrozole (Arimidex) on Intratumoral Estrogen Levels and Proliferation Markers in Patients with Locally Advanced Breast Cancer

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