What is pellet bio-identical hormone replacement therapy? Wha t are pellets? Each pellet implant is made up of a bio-identical hormone, either testosterone or estradiol, which is the exact chemical formula and molecular structure of what your body produces. They are derived from natural plants that are pressed into small solid cylinders, larger than a grain of rice and smaller than a Tic-Tac . In the United States, most pellets are made by compounding pharmacists since big