Assistant Chief Coroner Ed Winter said there's nothing unusual about this kind of request and that it's made in virtually all death investigations.
Winter said some doctors have already been contacted with requests for records.
Subpoenas have also been served at multiple locations, including Mickey Fine Pharmacy, the same Beverly Hills location where DEA agents sought files on Michael Jackson's prescriptions.
Dr. Richard Horowitz, a clinical professor of pathology at the USC Keck School of Medicine, says investigators want to know Houston's habits with drugs.
"Sometimes an individual that has a long history of drug addiction may take drugs to quiet themselves, and then if they're in the bathtub that may be just enough to make themselves woozy so that they drown," Horowitz said.
People close to Houston will likely be interviewed to find what may be the most significant information of all: her state of mind.
"Have there been any changes in her lifestyle? Have there been changes in stresses? Has there ever been a past suicide attempt?" Horowitz said.
Investigators found several bottles of prescription medication in the Beverly Hills hotel room where Houston died Saturday. Detectives have declined to say which medications were seized.
Authorities said an autopsy found no indications of foul play or obvious signs of trauma on Houston. She was under water and apparently unconscious when she was pulled from a bathtub, officials said.
It could be weeks before toxicology test results come back, but in the meantime, the coroner's office is looking for Houston's medical records to shed light on their investigation.
The singer had a public battle with drugs. In 2009, she admitted to abusing cocaine, marijuana and pills.
Houston died the day she was scheduled to perform at producer Clive Davis' pre-Grammy Awards bash. Her family plans a private church service in her hometown of Newark, N.J.
The pop icon was one of the world's best-selling artists from the mid 1980s to the late 1990s with hits that include, "I Wanna Dance With Somebody," "How Will I Know," "The Greatest Love of All" and her trademark song, "I Will Always Love You."
Houston's music's popularity has skyrocketed since her death, pushing her songs back on the charts and into heavy rotation on the radio.
See photos of Whitney Houston throughout the years.
The Associated Press contributed to this story.
