There was no autopsy performed but toxicology test results gave additional clarity, according to the medical examiner's office.
Trachtenberg's family objected to an autopsy for religious reasons.
The 39-year-old actress was found dead in her apartment in Columbus Circle in February.
Trachtenberg, a New York City native, made her acting debut as a child star with roles on Nickelodeon, including the television series "The Adventure of Pete & Pete" and the film "Harriet the Spy."
Her big break came when she was cast on "Buffy the Vampire Slayer" as Dawn, the younger sister of Sarah Michelle Gellar's titular character, from 2000 to 2003.
Trachtenberg starred as Georgina Sparks on "Gossip Girl" from 2008 to 2012. For her role, she was nominated as the choice TV villain at the Teen Choice Award in 2012.
(ABC News contributed to this report.)