The active formula in Blackstone's Letro XT™ contains a unique anti-aromatase ingredient,3b-hydroxy-androsta-4,6-diene-17-one. It's a compound developed by HiTech Pharmaceuticals and often referred to as the"ATD imposter"due to its structure and mechanism of action. This amazing compound found in Letro XT™ has a prolonged half life in the human body. This means you only take it once per day as opposed to many times per day like other anti-estrogens. This compound is a suicidal aromatase inhibitor because it permanently binds to the aromatase enzyme, thus preventing any sort of "estrogen rebound." It is best used for its aromatase inhibiting and testosterone boosting effect. Its effectiveness at lowering estrogen is unmatched.