A severely malnourished and abused 4-year-old girl whose body was found dead in her Brooklyn apartment last month — weighing just 18 pounds — died of drug poisoning, blunt-impact injuries, malnutrition and dehydration, officials said yesterday.

Marchella Pierce, whose death has been ruled a homicide, had antihistamines, including Benadryl and Claritin, in her system, sources said.

Her mother, Carlotta Brett-Pierce, 30, has already been charged with assault in connection with beatings administered to Marchella weeks before her death.