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Let’s start by stating that driving while impaired by drugs or alcohol is a crime and must be punished. All 50 US states have clear laws prohibiting this activity.

But there is one intoxicant that is trickier than the others: marijuana, especially when used for medical purposes.

During the past two years, Colorado and Montana, along with more than a dozen other states, have proposed laws that set a strict threshold for determining when a marijuana user is deemed too impaired to drive. These would consider a concentration of more than 5 nanograms of tetrahydrocannabinol, or THC (the psychoactive component of marijuana) per milliliter of blood, as hands-down proof of intoxication or impairment.

The result would be an automatic guilty verdict, with all that entails: a temporary loss of driving privileges, fines, lawyer’s fees, possible jail time and greatly increased insurance premiums. By some estimates, a conviction for driving under the influence (DUI) can cost a driver as much as $10,000.

Several states are going further and have either adopted or are considering zero-tolerance laws for THC levels. This means any THC in the blood would result in a conviction.

Here’s the problem with these laws: There are questions about how, and at what level, cannabis use impairs driving ability. For a patient in one of the 17 states where marijuana has been legalized for medicinal use, how are you to know when it’s legal to drive? After consuming marijuana, should you wait 12 hours to drive or one day? When will your THC level be below the 5-nanogram threshold? The answer is complicated.

Chronic users
Although marijuana is readily detectable in toxicology tests of blood, hair, urine or saliva, what isn’t clear is just how quickly THC passes through the body. We know, for example, that THC may be detected in the blood of occasional users several hours after ingesting. But in some chronic users there may be traces for days after the last use, long after any performance-impairing effects have subsided.

This is a very clear contrast with alcohol. There is a firm understanding of the rate at which the body metabolizes alcohol and there are well-known guidelines on how much time must pass after drinking before one is fit to drive. Tests can easily be administered in roadside stops. Those who fail simple benchmarks of sobriety -- not to mention breath tests -- are usually convicted or plead guilty.

The research on how marijuana affects driving is far less conclusive, though.

Testing done on drivers under the influence of alcohol often show that drivers display more aggressive behavior behind the wheel, and errors are more pronounced than when sober. The opposite tends to be true when drivers are under the influence of THC; they tend to have heightened awareness -- rather than diminished sensitivity as they do after drinking -- to their surroundings. As a result, they tend to compensate by driving more cautiously.

A 2007 control study published in the Canadian Journal of Public Health reviewed 10 years of U.S. auto-fatality data. Investigators found that U.S. drivers with blood-alcohol levels of 0.05 percent -- a level below the national 0.08 percent legal limit -- were three times as likely to have been driving unsafely before a fatal crash, compared with individuals who tested positive for marijuana.

What this means is that we need more research before new DUI marijuana laws are enacted. Setting an absolute impairment standard for THC bloodstream levels is premature. And these laws, which target marijuana use and associated medical marijuana patients, are discriminatory.

Pain killers
I say this at a time when there is an absence of legislation dealing with the use and well-documented abuse of prescription painkillers, which can dangerously impair the judgment needed for safe driving. State legislatures aren’t setting arbitrary and scientifically unproven blood-level standards for these drugs. So why are they focused on marijuana?

Driving while intoxicated must anywhere and everywhere be illegal, whether that impairment is caused by prescription drugs, alcohol purchased at a liquor store or marijuana used on the recommendation of a doctor. Under current standards, someone can be charged with DUI for marijuana use based on roadside sobriety tests and observations by the arresting officer in conjunction with blood samples. Those tests serve their purpose at this point.

But if states are going to turn to strict threshold laws, they should answer this question: Based solely on THC concentrations in blood from marijuana, when is a driver too impaired to drive safely?

Until the evidence is in, it’s hard to see why any state needs to lower the burden of proof necessary to convict someone of a DUI marijuana charge.