The Academy Award best actor winner for the lead role of Truman Capote in 2005's Capote was found in his New York Apartment on February 2.
The city's chief medical examiner said the 46-year-old's cause of death was due to acute drug intoxication from substances including heroin, cocaine, benzodiazepines and amphetamine.
It was reported at the time Hoffman was found with a needle in his body, while heroin was is dicovered in at least 50 packets in his Manhattan apartment.
Hoffman had spoken of his long battle with drugs when he was younger, but after 23 years sober revealed he had suffered from new substance abuse issues.
There were four arrests for possession of heroin following the tragedy. The lawyer for those who were held denied their clients were involved in his death.
The four-times Oscar nominated actor is the latest US celebrity to die from an overdose in recent years.
Glee actor Cory Monteith, 31, died in Vancouver of an accidental overdose of heroin and alcohol in October. Drugs also caused the death of actor Heath Ledger in 2008 and singer Whitney Houston in 2012.
Hoffman's death lead to an outpouring of grief among the acting profession on both sides of the Atlantic. Leading Hollywood stars attended his funeral and tributes were paid to the star at last month's Baftas in London.
He left behind a partner and three young children
Hoffman was nominated for a Tony after appearing in Arthur Miller's Pulitzer Prize-winning play Death of a Salesman, and was also nominated for roles in Long Day's Journey Into Night and True West.
His screen roles included The Master, Doubt" and Charlie Wilson's War for which he won best supporting Oscar nomination.
Heroin use has reached epidemic proportions in the US during the past five years.