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Possible causes of death

Lee's iconic status and untimely death fed many rumors and theories. These included murder involving the triads and a supposed curse on him and his family.

Donald Teare, a forensic scientist recommended by Scotland Yard who had overseen over 1,000 autopsies, was assigned to perform an autopsy on Lee. His conclusion was " death by misadventure " caused by cerebral edema due to a reaction to compounds present in the combination medication Equagesic. According to autopsy reports, Lee's brain had swollen from 1,400 to 1,575 grams, a 12.5% increase. Lee had taken Equagesic on the day of his death, which contained both aspirin and the tranquilizer meprobamate, although he had taken it many times before.

Although there was initial speculation that cannabis found in Lee's stomach may have contributed to his death, Teare said it would "be both 'irresponsible and irrational' to say that might have triggered either the events of Bruce's collapse on May 10 or his death on July 20". Dr. R. R. Lycette, the clinical pathologist at Queen Elizabeth Hospital, reported at the coroner hearing that the death could not have been caused by cannabis.

In a 2018 biography, author Matthew Polly consulted with medical experts and theorized that the cerebral edema that killed Lee had been caused by over-exertion and heat stroke; heat stroke was not considered at the time because it was then a poorly understood condition. Furthermore, Lee had his underarm sweat glands removed in late 1972, in the apparent belief that underarm sweat was unphotogenic on film. Polly further theorized that this caused Lee's body to overheat while practicing in hot temperatures on May 10 and July 20, 1973, resulting in heat stroke that in turn exacerbated the cerebral edema that led to his death.

In an article in the December 2022 issue of Clinical Kidney Journal, a team of researchers examined the various theories regarding Lee's cause of death, and concluded that his fatal cerebral edema was brought on by hyponatremia, an insufficient concentration of sodium in the blood. The authors noted that several risk factors predisposed Lee to hyponatremia, including excessive water intake, insufficient solute intake, alcohol consumption, and use or overuse of multiple drugs which impair the ability of the kidneys to excrete excess fluids. Lee's symptoms before his death were also found to closely match known cases of fatal hyponatremia.