Complex sentence: Due to the quantities of pills he was buying, it was believed he was trafficking the drugs, but a two-year investigation turned up no such evidence, and the quantities he bought are not unusual for long time users of opioid painkillers.
Split into simpler sentences: Due to the quantities of pills he was buying, it was believed he was trafficking the drugs. However, a two-year investigation turned up no such evidence, and the quantities he bought are not unusual for long time users of opioid painkillers.