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The sudden death of 28-year-old actor Heath Ledger, whose body was found Tuesday with sleeping pills nearby, has highlighted the dangers surrounding a drug class prescribed to millions of Canadians each year: sedatives.

An autopsy was inconclusive and more tests are needed to determine how the Australian-born former Oscar nominee died in his SoHo apartment, the New York medical examiner's office said yesterday.

But police reports that sleeping pills and anti-anxiety medications were found in the apartment - combined with a recent interview in which Mr. Ledger said he was using Ambien, a brand of sedative - suggest prescription drugs may have played a role.

While sleeping pills used alone are "reasonably safe," mixing those pills with other sedatives is a dangerous cocktail, said Jack Uetrecht, who holds a Canada Research Chair in immunotoxicology and whose research looks at adverse drug reactions.

"It's hard to kill somebody with Valium," said Dr. Uetrecht, professor of pharmacology and medicine at the University of Toronto. "But if you drink alcohol and take Valium, it's pretty easy to kill someone that way."

Sleeping pills belong to a class of drugs called benzodiazepines, which work by slowing down activity in the brain, or the central nervous system, while allowing a person to breath normally.

Today's pills, which include brands such as Valium or Ativan, are less likely to lead to accidental fatal overdoses than barbiturates of the past, Dr. Uetrecht said.

But dangers arise when those sedatives are mixed with other drugs that slow down the central nervous system, such as anti-anxiety medications or alcohol, Dr. Uetrecht said. The risk is that people will stop breathing, he said.

In rare cases, younger adults have died from heart failure after taking certain medications that conflict with a previously unknown genetic condition, Dr. Uetrecht said.

Heart complications may arise if a stimulant, such as cocaine, is used in combination with prescription sedatives, Dr. Uetrecht said, but "it's hard to speculate."

A rolled-up $20 (U.S.) bill was found in Mr. Ledger's apartment, police said yesterday, but it had not yet been tested for traces of illegal substances.

People in this spent $123-million (Canadian) on prescription sedatives in 2006, the most recent year that data is available from IMS Health Canada, a private company that tracks prescription drug sales. That year, almost six million prescriptions for sedatives were dispensed from Canadian pharmacies, about 1 per cent of all prescriptions. That was up from about 4.7 million in 2002.

Research shows that sedatives can also be addictive. People's bodies begin to acclimatize to the pills' effects in a matter of weeks, Mr. Uetrecht said, but when they stop using the pills, "then you have real difficulty sleeping." A Toronto study released last summer found that about half of elderly patients prescribed common medications to help them sleep are still taking the drugs at least six months after discharge.

"I don't think, in general, they should be used except for very short periods of time," Dr. Uetrecht said. "Because, ultimately, I think they make sleep worse."