Feb. 27, 2012 -- A provocative new study finds that people who take prescription sleeping pills -- even once in a while -- have a higher death risk than non-users.

The top third of sleeping-pill users had a 5.3-fold higher death risk. They also had a 35% higher risk of cancer, the study found.

"We are not certain. But it looks like sleeping pills could be as risky as smoking cigarettes. It looks much more dangerous to take these pills than to treat insomnia another way," study leader Daniel F. Kripke, MD, tells WebMD.

The sleeping pills in question are known as hypnotics. They include newer drugs such as zolpidem (the best known brand name is Ambien) as well as older drugs such as temazepam (the best known brand name is Restoril).

Hypnotic sleeping pills actually cause a person to fall asleep. This sets them apart from other sleeping aids, such as the supplement melatonin, which promote sleep through relaxation. Other sleep drugs described as hypnotics by Kripke and colleagues include eszopiclone (Lunesta), zaleplon (Sonata), triazolam (Halcion), flurazepam (Dalmane), barbiturates, and older antihistamines such as diphenhydramine.

Kripke, emeritus professor of psychiatry at the University of California, San Diego, began looking at a possible link between sleeping pills and death risk in 1975. Since then, he and others have published 18 studies that found a link.

In their latest study, Kripke's team analyzed 2002-2007 data from a large Pennsylvania health system. They obtained medical records for 10,529 people who were prescribed hypnotic sleeping pills and for 23,676matched patients who were never prescribed sleeping pills.

Over an average of 2.5 years, the death rate for those who did not use sleeping pills was 1.2%. It was 6.1% for people with sleeping pill prescriptions. Even those prescribed 18 or fewer sleeping pills a year had a 3.6-fold higher death risk.

Based on these findings, Kripke and colleagues estimate that sleeping pills are linked to 320,000 to 507,000 U.S. deaths each year.

"We think these sleeping pills are very dangerous. We think they cause death. We think they cause cancers," Kripke says. "It is possible but not proven that reducing the use of these pills would lower the U.S. death rate."