Many states are confronting the problem as part of a broader effort to keep so-called drugged drivers, including those under the influence of marijuana and other illegal drugs, off the road.

“We have a pretty clear message in this country that you don’t drink and drive,” said R. Gil Kerlikowske, President Obama’s top drug policy adviser, who wants to reduce drugged-driving accidents by 10 percent over the next five years. “We need very much to have a similar message when it comes to drugs.”

There is no reliable data on how many drivers are impaired by prescription drugs, but law enforcement officials say the problem is growing so quickly that states are putting hundreds of police officers through special training to spot signs of drug impairment and clamoring for better technology to detect it.

Even the prevalence of drug-impaired driving is unknown, since many states combine the arrest data with that for drunken driving. Mr. Kerlikowske points to a 2007 survey by the National Highway Traffic Safety Administration, which screened 5,900 nighttime drivers around the country and found that 16.3 percent tested positive for legal or illegal drugs.

The tests could not determine which drivers were impaired by drugs, but Mr. Kerlikowske said the results suggested a problem that had “flown below the radar” for too long.

“You don’t want to scare people,” he said, “but you certainly want to make them aware of the dangerousness. You can be as deadly behind the wheel with prescription drugs as you can with over-the-limit alcohol, and you are responsible for your own actions.”

In interviews, law enforcement officials around the country said anyone who drives while taking prescription drugs is at risk of arrest, not only those who drive recklessly. In one recent case near Bangor, Me., a pickup truck on a rural road was not swerving, speeding or otherwise hinting that its driver was impaired. A police officer stopped the truck because of its noisy muffler, then saw that the driver’s eyes were bloodshot and his speech slurred.