By ABBY GOODNOUGH and KATIE ZEZIMA


Prescription drug use is rampant in the United States, and, unlike with alcohol, there is no agreement on what level of drugs in the blood impairs driving. The issue is vexing police officials nationwide.

The behavioral effects of prescription medications vary widely, depending not just on the drug but on the person taking it. Some can dull alertness and slow reaction time; others can encourage risk-taking and hurt the ability to judge distances. Mixing prescriptions, or taking them with alcohol or illicit drugs, can exacerbate impairment , researchers say.

The police also struggle with the challenge of prosecuting someone who was taking valid prescriptions.

“How do we balance between people who legitimately need their prescriptions and protecting the public?” said Mark Neil, senior lawyer at the National Traffic Law Center .

Setting limits for prescription medications is complicated, partly because the chemistry of drugs makes their effects more difficult to predict than alcohol’s. Some drugs can linger in the body for days or weeks.

“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 . “We need very much to have a similar message when it comes to drugs.”

Officials say the problem is growing so quickly that states are training hundreds of police officers to spot signs of drug impairment and clamoring for better technology to detect it.

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 drivers around the United States and found that 16.3 percent tested positive for legal or illegal drugs.

“You don’t want to scare people,” Mr. Kerlikowske said, but you want to make them aware of the danger . “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.”

Law enforcement officials said anyone who drives while taking prescription drugs is at risk of arrest, not only those who drive recklessly.

In Wisconsin, a former physician slammed his sport utility vehicle into a Honda Accord in April 2008, killing the pregnant driver and her 10-yearold daughter. Prosecutors said the physician had high levels of the sleep aid Ambien in his system, as well as Xanax, an anti-anxiety drug, and oxycodone, an opiate painkiller. He was sentenced to 30 years in prison.

Defendants can try to prove that they did not realize their medication would affect their driving, prosecutors said, but that argument may not hold if the bottle had a warning label.

“Would you go home and start a chain saw and cut down a tree?” said Lieutenant Colonel Thomas C. Hejl, the assistant sheriff in Calvert County, Maryland. “Why should you get behind the wheel of a vehicle when the same medication has the same side effects?”

Prosecutors in drugged-driving cases rely heavily on the testimony of “drug recognition experts,” law enforcement officers trained to spot signs of impairment in drivers. But there are only about 7,000 such officers nationwide.

And persuading a jury to convict someone of impaired driving due to prescription drugs remains difficult except for the most egregious cases, said Douglas F. Gansler, the attorney general in Maryland.

“Because most people on the jury will also likely be taking prescription drugs for some ailment,” Mr. Gansler said, “whether it’s Lipitor or allergy pills or whatever it might be, they might think, ‘I don’t want that to become criminal.’ ”