Creatine

Creatine is a nitrogenous acid produced from amino acids in the liver and kidneys. It is carried by the bloodstream to the skeletal muscles of the human body (or other animal's body), where 95% of it is normally found. The other 5% of creatine is used in the heart and brain.

As it is manufactured by the body, creatine is not an essential nutrient, but it is commonly supplemented for the purpose of increasing muscle growth. The components that the body requires to make it are found largely in meat products, so it is especially needed by vegetarians if they wish to grow muscle mass. Some doctors suspect that it may influence insulin tolerance, so creatine is not recommended for people with diabetes or hypoglycemia.

Creatine has shown some promise against diseases of muscular degeneration.  From a sexual standpoint, since creatine allows your muscles to work longer before you feel pain or fatigue, it improves performance and for that reason it is an ingredient in many male enhancement formulas.