LOS ANGELES – It’s become standard-operating procedure when a celebrity dies too young – investigators immediately go looking through their nightstand and medicine cabinet.

That effort is well under way in the death of Whitney Houston, with investigators saying Wednesday they have subpoenaed records from the singer’s doctors and pharmacies who dispensed medication found in her hotel room.

The inquiries are routine in virtually all death investigations, Assistant Chief Coroner Ed Winter said, noting that similar measures are taken when a person dies in a car crash, shoots himself or, as in Houston’s case, dies unexpectedly.

It will be weeks before toxicology results reveal the medications and quantities, if any, that were in Houston’s system when she died. The Grammy winner’s history of substance abuse has added to the speculation that her death may have been caused by prescription drugs.

Abuse of prescribed medications has skyrocketed in recent years. In 2008, more than 36,000 people died from drug overdoses – triple the number from 1990 – with most of these deaths caused by prescription drugs.

Drug deaths, fueled by prescription drug overdoses, now surpass motor vehicle deaths, according to the Centers for Disease Control and Prevention.

Investigators have not said what medications they have recovered from Houston’s room at the Beverly Hilton Hotel. The singer was found underwater in a bathtub by a member of her staff hours before she planned to attend a chic pre-Grammy gala. Police have said there were no signs of foul play and Winter said there were no signs of trauma on her body when an autopsy was conducted on Sunday.

Among the scenarios that will likely be explored is whether Houston was drinking before her death, which could compound the effects of any medications she was taking.

“Sometimes people fall into a stupor when they’re on a combination of drugs so they’re difficult to arouse,” said clinical psychiatrist and addiction specialist Dr. Karen Miotto at the University of California, Los Angeles.

Sober people who fall asleep in the bathtub will wake up when water hits their face. An impaired person may not respond the same way.

Even a small amount of prescription drugs combined with alcohol “can result in a state of unconsciousness and inability to rescue oneself from drowning in the bathtub,” said Bruce Goldberger, a forensic toxicologist at the University of Florida.