Letrozole is currently the most powerful aromatase inhibitor available. It has been shown to reduce estrogen levels by 98% or more. Intravenous administration of Letrozole lowered Estrogen by 46% in the young men tested, and 62% in the elderly subjects. Because estrogen is part of the negative feedback loop of the HPTA, Letrozole (and other anti-estrogens) are able to raise testosterone in male subjects. Letrozole was studied in men, and found to significantly increase LH levels to a 339 and 323% in the young and the elderly, respectively and Testosterone by 146 and 99%, respectively.

Letrozole was also able to produce a peak LH response to Gonadatropin Releasing Hormone equal to a 152 and 52% increase from baseline in either young or older men, respectively. In a similar study 0.02 mg of Letrozole increased testosterone by 45% after 2 days. That same twenty micrograms of Letrozole was also enough, in one study done on men, to reduce estrogen levels by roughly a third. Letrozole has a 2-4 day half-life, and it needs to be taken for up to 60 days to get a steady blood plasma level. Letro is best dosed from 0.125-2.5 mgs depending on the desired effect.