The original coroner's report said Louden choked to death on his own vomit because he had alcohol and papaverine, an erectile dysfunction drug, in his system. The teen's family has disputed the report ever since it was revealed.
The new findings show the boy was injected the drug papaverine just after his death when he was being prepped to be a tissue donor, Paula Valle, of the California Donor Network said.
The boy's family received a letter from the tissue transplant bank, saying staff injected Joseph with the drug right after his death.
Joseph, called "J-Lo" by friends, was a sophomore and star athlete at the school.
Police have arrested two people in connection with his death.