Zombie drivers — motorists driving under the influence of the popular prescription sleeping pill Ambien, possibly while sleepwalking and unaware they are doing so — are a rising threat on U.S. roads, according to a report published in the New York Times Wednesday.

According to the Times, ten state toxicology labs that test for the presence of the drug rate Ambien among the top 10 drugs found in impaired drivers. In Wisconsin, Ambien was identified in the bloodstreams of 187 drivers arrested between 1999 and 2004, the newspaper reported. In Washington State, Ambien was found in 78 impaired drivers arrested in 2005, up from 56 the previous year.

Following their arrests, many of the Ambien drivers claim to have no recollection of getting behind the wheel, according to the report. In many of the cases the drug appears to have been taken incorrectly, combined with alcohol or other drugs or taken in overdose quantities; several of the cases suggest, however, that even when the drug is used properly, a sleepwalking side effect may lead to sleep driving, the Times reported.

In 2004, the FDA received 48 reports of “adverse events” involving Ambien without the use of other drugs, the Times reported. Those events included three sleepwalking cases, a traffic accident and six reports of hallucinations, the newspaper said.

The bizarre behavior of drivers found to be under the influence of Ambien also may suggest a direct relation to the drug, experts told the Times. The report recounted incidents of motorists driving down the wrong side of the road, urinating in the street, driving directly into objects and becoming violent with the police attempting to apprehend them.

However, the Times report also suggested broad misuse and disregard for side effect warnings among Ambien users. Ambien comes with a label warning of side effects including sleepwalking and hallucinations, and warns against combining the drug with alcohol. Yet, in many of the “Ambien driver” cases, alcohol was a factor, according to the report.

A spokesperson for Ambien manufacturer Sanofi Aventis told the Times that the drugmaker was aware of the sleepwalking cases and forwarded those reports to the FDA as part of the company’s ongoing evaluation of the drug’s safety.

A spokesperson for the FDA told the Times that the drug’s label warning of side effects and alcohol use were “sufficient” and that Ambien users needed to be more careful in using the drug properly.

Ambien is the number one sleeping pill in the United States, with 26.6 million prescriptions and $2.2 billion in sales last year, the Times reported.