Complex sentence: The Sinclair method is a method of using opiate antagonist, such as naltrexone, to treat alcoholism by having the person take the medication about an hour before they drink alcohol, and only then, in order to avoid side effects that arise from chronic use.
Split into simpler sentences: The Sinclair method is a method of using opiate antagonists such as naltrexone to treat alcoholism. The patient takes the medication about an hour (and only then) before drinking to avoid side effects that arise from chronic use.