The primary cause of death was pneumonia, with contributing factors of iron deficiency anemia and "multiple drug intoxication," according to the Los Angeles County coroner's office.
The death was ruled accidental.
Coroner's spokesman Craig Harvey declined to specify what types of drugs were involved but said they were all prescription medications. Details about the drugs the "Clueless" and "8 Mile" star had in her system will be released in several weeks when a complete report is done.
Murphyat age 32 after collapsing at her Hollywood Hills home. Her mother and husband have said the actress didn't abuse prescription medications or have an eating disorder.
Attempts to reach them for further comment were not immediately successful Thursday.
Murphy's husband, Simon Monjack, and mother, Sharon Murphy, told investigators the actress had been experiencing flulike symptoms in the days before she died.
Monjack told The Associated Press last month that his wife did take several prescriptions, including an anti-seizure drug, but did not abuse the medications.
He said she had been taking over-the-counter Robitussin for her recent sickness.
A memorial for a foundation in Brittany Murphy's name that had been planned for Thursday was abruptly canceled with no explanation.
Dr. Michael Baden, the former chief medical examiner in New York City, said the coroner's determination indicated drugs probably played a key role in Murphy's death.
"If she died of pneumonia, that cause of death is a natural cause," Baden said. He said the use of some prescription medications can lead to pneumonia, and that the coroner's office may have listed that as the primary cause to lessen the blow to Murphy's family.
"It sounds more like it's a drug-related than a natural pulmonary pneumonia," said Baden, who did not work on the Murphy case but has served as an expert witness in numerous high-profile deaths.
He said the quantities and types of drugs Murphy was taking is key to understanding how she died, as well as how recently she saw a doctor.
"One doesn't die of pneumonia, usually, that quickly," Baden said. He also downplayed the role anemia may have played in Murphy's death.