Carter, 34, the younger brother of the Backstreet Boys' Nick Carter, was found dead at his home in Lancaster, California, on 5 November.
The autopsy has now revealed that his death was caused by drowning due to the effects of sedatives he had taken and gas he had inhaled.
Alprazolam, often sold under the brand name Xanax, was found in his system.
The compressed gas difluoroethane was also detected. The report describes it as a "gas commonly used as a propellant in air spray cleaners" which "can induce feelings of euphoria when inhaled".
