If they found positive toxicology results as the likely cause, they will try to determine whether caused by therapeutic, accidental or self inflicted OD.
Or murder.
Interesting that L Ron Hubbard didn't get an autopsy, and was promptly cremated, in accordance with scientology beliefs (Which, BTW, came as a total surprise to the rest of us in scientology). He was under the care of the now deceased Dr Gene Denk.
Dead men tell no tales...