Hypnotic effects
There is no biological pathway that has been proven to connect zolpidem with these behaviours. The drug is a benzodiazepine-like hypnotic that promotes deep sleep by interacting with brain receptors for a chemical called gamma-aminobutyric acid. While parts of the brain become less active during deep sleep, the body can still move, making sleepwalking a possibility.
The product information for prescribers advises that psychiatric adverse effects, including hallucinations, sleepwalking and nightmares, are more likely in the elderly, and treatment should be stopped if they occur.
Patient advocacy groups say they would like government health agencies and drug companies to take a closer look at the possible risks associated with sleep medicines. They stress that strange sleepwalking and sleep-driving behaviours can have risky consequences.
“When people do something in which they’re not in full control it’s always a danger,” says Vera Sharav of the New York-based Alliance for Human Research Protection, a US network that advocates responsible and ethical medical research practices.
Tried and tested
“The more reports that come out about the potential side effects of the drug, the more research needs to be done to understand if these are real side effects,” says sleep researcher Kenneth Wright at the University of Colorado in Boulder, US.
Millions of people have taken the drug without experiencing any strange side effects, points out Richard Millman at Brown Medical School, director of the Sleep Disorders Center of Lifespan Hospitals in Providence, Rhode Island, US. He says that unlike older types of sleep medications, zolpidem does not carry as great a risk of addiction.
And Wright notes that some of the reports of “sleep-driving” linked to zolpidem can be easily explained: some patients have wrongly taken the drug right before leaving work in hopes that the medicine will kick in by the time they reach home. Doctors stress that the medication should be taken just before going to bed.
The US Food & Drug Administration says it is continuing to "actively investigate" and collect information about cases linking zolpidem to unusual side effects.
The Ambien label currently lists strange behaviour as a “special concern” for people taking the drug. “It’s a possible rare adverse event,” says Sanofi-Aventis spokesperson Melissa Feltmann, adding that the strange sleepwalking behaviours “may not necessarily be caused by the drug” but instead result from an underlying disorder. She says that “the safety profile [of zolpidem] is well established”. The drug received approval in the US in 1993.