Third, GH may influence the activity of IGF-1 on an autocrine/paracrine level. Autocrine means that a hormone has an effect on the cell that produced it, paracrine means to have an effect on the “cell(s)” next to it as well. This is a completely localized effect, not dependent on the blood stream to carry things where you want them. Muscle growth from weight training is the result of IGF-1 being produced by the muscle cells themselves, not the liver. In fact, IGF-1 form the liver is genetically different from IGF-1 produced in your muscles. This information should explain why using IGF-1 systemically (from the blood stream) has been a hit and miss proposition.
Some of the approved drugs are synthetic versions of the natural hormones, such as trenbolone acetate and zeranol. Just like the natural hormone implants, before FDA approved these drugs, FDA required information and/or toxicological testing in laboratory animals to determine safe levels in the animal products that we eat (edible tissues). Furthermore, FDA required that the manufacturers demonstrate that the amount of hormone left in each edible tissue after treatment is below the appropriate safe level. As described above, a safe level is a level which would be expected to have no harmful effect in humans.