Complex sentence: Chopra said one of the reasons for leaving mainstream medicine was his disenchantment at having to prescribe too many drugs, since in his view "80 percent of all drugs prescribed today are of optional or marginal benefit".
Split into simpler sentences: Chopra said one of the reasons for leaving mainstream medicine was his disenchantment at having to prescribe too many drugs. According to Chopra "80 percent of all drugs prescribed today are of optional or marginal benefit".