(New York Times) -A large study has linked several common anti-anxiety drugs and sleeping pills to an increased risk of death, although it’s not certain the drugs were the cause.

For more than seven years, researchers followed 34,727 people who filled prescriptions for anti-anxiety medications like Valium and Xanax, or sleep aids like Ambien, Sonata and Lunesta, comparing them with 69,418 controls who did not.

After adjusting for a wide variety of factors, the researchers found that people who took the drugs had more than double the risk of death. The study appears online in BMJ.

The researchers tried to account for the use of other prescribed drugs, age, smoking, alcohol use, socioeconomic status, and other health and behavioral characteristics. Most important, the investigators also controlled for sleep disorders, anxiety disorders and other psychiatric illnesses, all of which are risk factors for mortality.

The lead author, Dr. Scott Weich, a professor of psychiatry at the University of Warwick, said that while he and his colleagues were careful to account for as many potential risks as possible, they were not able to control for the severity of the illnesses suffered by the study participants.