Kerry Shoesmith agreed to send Peter Hamer diamorphine which she got from a friend, a court was told.
Shoesmith, 35, of East Street, Okehampton, Exeter, pleaded guilty to supplying class A drugs.
Prosecutor, Jim Mowbray, said the defendant had posted a bottle of morphine, which she got from a friend, to her step-father, who lived in Blackpool, on January 20.
She sent the drugs to her step-father as a result of speaking to him and agreeing to send them to him.
The prosecutor added: “As a result of ingesting the morphine, and possibly other substances, her step-father subsequently died.”
Shoesmith, who has no previious convictions, was bailed to appear at Preston Crown Court for sentence on July 27.