Complex sentence: Barbiturates such as phenobarbital were long used as anxiolytics and hypnotics, but today have been largely replaced by benzodiazepines for these purposes because of less potential for lethal overdoses.
Split into simpler sentences: Barbiturates such as phenobarbital were long used as anxiolytics and hypnotics. Today, benzodiazepines have largely supplanted them for these purposes, because benzodiazepines have less potential for lethal overdoses.