DENVER -- Angeline Chilton says she can't drive unless she smokes pot.

The suburban Denver woman says she'd never get behind the wheel right after smoking, but she does use medical marijuana twice a day to ease tremors caused by multiple sclerosis that previously left her homebound.

"I don't drink and drive, and I don't smoke and drive," she said. "But my body is completely saturated with THC."

Her case underscores a problem that no one's sure how to solve: How do you tell if someone is too stoned to drive?

States that allow medical marijuana have grappled for years with determining impairment levels. Voters in Colorado and Washington state will decide this fall whether to legalize pot for recreational use, adding urgency to the issue.

"The explosion of medical marijuana patients has led to a lot of drivers sticking the [marijuana] card in law enforcement's face, saying, 'You can't do anything to me, I'm legal,' " said Sean McAllister, a Denver lawyer who defends people charged with driving under the influence of marijuana.

It's not that simple. Driving while impaired by any drug is illegal in all states.

But it highlights the challenges law enforcement faces using old tools to try to fix a new problem. Most convictions for drugged driving now are based on police observations, followed by a blood test.

Authorities envision a legal threshold for pot comparable to the blood-alcohol standard to determine drunken driving. But unlike alcohol, marijuana stays in the blood long after the high wears off a few hours after use. There is no quick test to determine someone's level of impairment -- not that scientists haven't been working on it.

Dr. Marilyn Huestis of the National Institute on Drug Abuse, a government research lab, says there will soon be a saliva test to detect recent marijuana use. But government officials say that doesn't address the question of impairment.

"I'll be dead -- and so will lots of other people -- from old age, before we know the impairment levels" for marijuana and other drugs, said White House drug czar Gil Kerlikowske.

Authorities recognize the need for a solution. Marijuana causes dizziness, slowed reaction time and drivers are more likely to drift and swerve while they're high.

Chilton, the MS patient, says that, using pot, she has started driving again and for the first time in five years has landed a job. She worries that Colorado's proposal jeopardizes her newfound freedom.

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