A new study has raised concern for the estimated 50 to 70 million Americans who use sleeping pills.

People who used sleeping pills over a two-year period are three-and-a-half times more likely to die from all causes, according to the study published in the British Medical Journal.

Doctors say that could be because insomnia is often a symptom of something more serious.

"Sedation worsens sleep apnea, for example, and we know sleep apnea is associated with risk of death," Dr. Lee Green, a professor in the department of family medicine at the University of Michigan, told ABC News.

"We tend to think that a sleeping pill once in a while is harmless, but there's no such thing as a medication free of risk," he added.

Insomnia can be a warning sign of heart disease, diabetes, and even cancer.

Doctors warn that taking sleeping pills for insomnia might actually keep patients from learning of more serious conditions.