Mesterolone is a modified form of dihydrotestosterone. It differs by the addition of a methyl group at carbon 1, which helps protect the hormone from hepatic metabolism during oral administration. The same structural modification is also used with methenolone (Primobolan) tablets. Alkylation at the one position slows hepatic metabolism of the steroid during the first pass, although much less profoundly than c-17 alpha alkylation. Mesterolone is resistant enough to breakdown to allow therapeutically beneficial blood levels to be achieved, although the overall bioavailability remains much lower than c-17 alpha alkylated oral steroids. Mesterolone also has a very strong binding affinity for Sex Hormone Binding Globulin. This mat act to displace other steroids more weakly bound to SHBG into a free (active) state.