Matthew Perry had enough ketamine in his system that he could have been rendered incapacitated or unconscious, causing him to drown ... based on autopsy findings.
According to the L.A. County Medical Examiner, Perry's ketamine levels were 3,540 ng/ml. When surgery patients are given anesthesia, ketamine levels are in the 1,000 - 6,000 range, so Perry was in the higher range.
In other words, the amount of ketamine in his system was equivalent to the anesthesia some patients get before surgery. Perry could have just become disoriented enough that he drowned.
According to the M.E., Perry had been prescribed ketamine for depression and anxiety, although it is also used as a recreational drug. He was taking it every other day, but 6 months ago a new doctor tried to ween him off that regiment because he had showed signs of improvement.
The last prescribed dose was a week and a half before his death, and the effects generally only last around 6 hours. So it appears the 54-year-old actor may have taken the ketamine shortly before getting in the hot tub.