Every animal, including humans, has a particular hormone that is excreted to promote rapid growth, metabolism, muscle enhancement and all the other good stuff. It is basically a secret super weapon we have evolved to be able to adapt to the environmnet quickly. It makes growth happen and the young turn into full powered adults.

In the wild, animals that have little to do with their offspring exhibit this behavior very much. Giraffes, for example, give birth and their babies mature from birth to near adult size in the first 48 hours! This amazing growth spurt is spurned on by their growth hormone, and allows amazing things to happen.

In humans, human growth hormone is responsible for the same effects but only over a longer period of time beginning in puberty and ending in early adulthood. Many athletes as of recently have come under scrutiny for using human growth hormone to enhance physical performance, but there is great debate whether human growth hormone should be illegal since it is a natural substance.

As of now there is no conclusive research to show the longterm side effects of human growth hormone usage other than what your body produces, so it is uncertain whether this is a good or bad thing.