S-Adenosyl Methionine (SAMe) is the amino acid methionine bound to an ATP molecule; this molecule circulates in the blood naturally and acts as a 'methyl donor'. A methyl group in chemistry is simply a carbon molecule (bound to some hydrogens), and donating a methyl group to other molecules can accelerate or preserve reactions in the body as a form of metabolic 'maintenance'. Choline is another prominent methyl donor in the body, but due to selectivity of some reactions the two are not necessarily interchangeable.
Some body states appear to be associated with lower circulating levels of SAMe, including osteoarthritis and depression where SAMe appears to hold potent therapeutic potential; after a build-up period of 1-2 months, supplemental SAMe at 800-1600mg daily appears to be as potent as some pharmaceutical options for both osteoarthritis and depression and appears to enhance some anti-depressants. Although lower circulating levels of SAMe are also seen in diabetes, the benefits of SAMe and glucose control are not as well established as depression and osteoarthritis.
SAMe appears to follow many nutrient-like motifs in the body, being regulated within a certain serum range and being associated with adverse effects when that range is perturbed. It potentially holds a lot of benefit in body states where circulating SAMe levels decline.
Although side-effects are not commonly reported with SAMe, numerous studies note a small set of participants who experience mania after supplementing SAMe. It is not common, but it does appear to be related to SAMe supplementation for unknown reasons; it has been reported in some persons without history of mania as well and does not appear to be related to any pathological condition per se.