Lisa Robin Kelly Death Certificate -- 'Multiple Drugs' -- 'Oral Ingestion'

"That 70s Show" actress Lisa Robin Kelly died from "multiple drug intoxication" ... and her death certificate reveals .... she took an overdose of drugs by mouth.

The L.A. County Coroner's report -- obtained by TMZ -- confirms what we previously reported ... that Kelly's death was an accidental overdose.

Kelly -- who played Eric Forman's older sister on the show -- died August 15th at an alcohol rehab facility in L.A. County.

It's unclear how she obtained drugs in the rehab facility.