Testosterone esters have increasingly been used in replacement therapy, but abuse of these compounds has risen as well. A feature that all testosterone esters have in common is a testosterone molecule with a carboxylic acid group (ester linkage) attached to the 17-beta hydroxyl group. These esters differ in structural shape and size; they function only to determine the rate at which the testosterone is released from tissue. Generally, the shorter the ester chain, the shorter the drug's half-life and quicker the drug enters the circulation. Longer/larger esters usually have a longer half-life and are released into the circulation more slowly. Once in the circulation, the ester is cleaved, leaving free testosterone.