Final answer:
Naltrexone is an opioid antagonist that may result in weight loss.
Step-by-step explanation:
An opioid antagonist that may result in weight loss is naltrexone. Naltrexone is a narcotic antagonist that is well absorbed orally and is long-acting. It is used in narcotic treatment programs to maintain individuals on chronic therapy with a narcotic antagonist. In individuals taking naltrexone, subsequent injection of an opiate will produce little or no effect.