Anastrozole blocks aromatase, the enzyme that turns testosterone into estradiol. It is approved for breast cancer in women. In men it is used off-label to lower estradiol during testosterone therapy and to raise testosterone in some infertile men. The male evidence is small and mostly observational.
What it does
By blocking aromatase, anastrozole lowers estradiol and removes the feedback that suppresses the pituitary. The pituitary then sends more LH and FSH, and testosterone can rise.
The weight-loss trial
Colleluori and colleagues randomized men with obesity and hypogonadism to anastrozole or placebo, both with a weight-loss programme. After six months, the anastrozole group had higher testosterone (p 0.003) and lower estradiol (p 0.001). Symptoms and muscle strength did not differ, and the anastrozole group lost more fat mass (Front Endocrinol 2020).
Against clomiphene
Helo and colleagues compared clomiphene and anastrozole in hypogonadal infertile men. Testosterone rose by 571 ng/dL with clomiphene and 408 ng/dL with anastrozole. Estradiol fell with anastrozole and rose with clomiphene, and the testosterone-to-estradiol ratio rose significantly only with anastrozole. Semen parameters did not differ significantly (J Sex Med 2015).
Infertility
A 2021 review of clinical use of aromatase inhibitors in male infertility reports increased testosterone and FSH and improved semen parameters in infertile men. The authors describe the studies as clinical but small and mostly observational (Hum Reprod Update 2021).
Why estradiol is not a free benefit
In adult men, estradiol is more closely linked to bone density, bone turnover and fracture risk than testosterone is. A review found a threshold below which the male skeleton is impaired, so lowering estradiol too far is a bone risk (Vandenput and Ohlsson, Nat Rev Endocrinol 2009).
What this does not say
- No large randomized trial in men on testosterone shows that anastrozole improves symptoms or clinical outcomes.
- The weight-loss trial was not done in men on testosterone alone, and its numbers come from a single study.
- Estradiol targets vary between clinics; the level, not the pill, is what should be tracked.
How we use this
We use anastrozole only when estradiol is high enough to need it, and we track estradiol and testosterone, because the bone trade-off is real. We do not add it by default to testosterone therapy.
We do not promise that anastrozole improves fertility or symptoms. The evidence is limited, and we say so.
Explore weight loss treatmentsSources
- 1.Colleluori G et al. Aromatase inhibitors plus weight loss improves the hormonal profile of obese hypogonadal men without causing major side effects. Front Endocrinol 2020. Frontiers in Endocrinology
- 2.Helo S et al. A randomized prospective double-blind comparison trial of clomiphene citrate and anastrozole in raising testosterone in hypogonadal infertile men. J Sex Med 2015. Journal of Sexual Medicine
- 3.Yang C, Li P, Li Z. Clinical application of aromatase inhibitors to treat male infertility. Hum Reprod Update 2021. Human Reproduction Update
- 4.Vandenput L, Ohlsson C. Estrogens as regulators of bone health in men. Nat Rev Endocrinol 2009. Nature Reviews Endocrinology