It's a coenzyme (NADH) that has been tested in double blind studies, and shown effective for patients with chronic fatigue. Its a form of vitamin B-3. I have CFS, and have been taking ENADA for years now. It has helped me quite a bit.
Sufferers of Chronic Fatigue Syndrome (CFS) can be heartened by the favorable results of clinical trials of the nutritional supplement enada NADH for use in the treatment of their debilitating disorder. The outcome of the enada NADH study has recently been published in the respected medical journal Annals of Allergy, Asthma & Immunology. (Vol. 82, pp. 185-191, Feb.1999). Healthwatch and The CFIDS & FM Health Resource published preliminary results of this study, which was conducted at the prestigious Georgetown University Medical Center in Washington, D.C. This trial marks one of the first times the FDA has approved a nutritional supplement for evaluation as a medical treatment.
http://www.ImmuneSupport.com/library/showarticle.cfm/ID/96/T/CFIDS_FM/searchtext/enada/