Both words, adjectives derived from
anorexia meaning 'obsessive loss of appetite', are in use. but
anorexic is far more common and will eventually oust
anorectic completely. It is also used absolutely as a quasi-noun. Examples:
He became listless, anorexic, and increasingly sleepy, refusing to eat or crawl—Lancet, 1961
Contrary to the popular image of the disturbed teenager, the anorexic is not typically a product of a 'broken home'—S. Macleod, 1989
Do they think I'm anorexic, or just plain thin?—J. Dawson, 1990.