According to The Sun:

An Emergency Room source at UCLA hospital said Jackson aides told medics he had collapsed after an injection of potent Demerol — similar to morphine.
A Jacko source said: “Shortly after taking the Demerol he started to experience slow shallow breathing.
“His breathing gradually got slower and slower until it stopped.
“His staff started mouth-to-mouth and an ambulance was called which got there in eight minutes “Butfound he was in full respiratory arrest, no breathing and no pulse. They started full CPR and rushed him to hospital.