This natural substance found throughout the body is synthesized from the amino acid methionine, with the help of folic acid and vitamin B12. Tissue levels of this substance are typically low in the elderly and in individuals suffering from depression, osteoarthritis and various liver disorders. As a natural dietary supplement, this substance has been credited with slowing and even reversing the progression of osteoarthritis, relieving depression, and decreasing alcohol-induced liver damage. In fact, this substance has been used as a prescription drug in Europe for over 20 years for both depression and osteoarthritis, and preliminary research shows that this substance may provide beneficial effects for fibromyalgia. What is this natural dietary supplement?