Denver, CO-Yesterday was the first day that Colorado residents were legally able to purchase marijuana for recreational use. As snow fell, people lined up at dispensaries across the state to purchase all manner of cannabis treats from buds to pot-lace brownies and candies. Many of those who lined up to purchase weed were from states across the country, ready to legally purchase marijuana.

While the laws governing marijuana use are becoming increasingly progressive throughout the U.S. –some feel it’s only a matter of time before other states follow in the footsteps of Washington and Colorado and make marijuana legal for recreational use– the laws pertaining to driving under the influence of marijuana are regressive and not necessarily based on science.

Although marijuana is widely used, there have been too few studies to determine the effects it has on an individual’s ability to drive. Even the National Highway Traffic Safety Administration is not clear on how marijuana affects a driver’s abilities stating on their website, “It is difficult to establish a relationship between a person’s THC blood or plasma concentration and performance impairing effects … It is inadvisable to try and predict effects based on blood THC concentrations alone.”

Despite the lack of conclusive empirical evidence, the DUI laws pertaining to marijuana can be somewhat draconian and can land people in trouble even when they aren’t under the influence.

In the U.S. there are at least 11 states where a person can be charged with driving under the influence of marijuana even if they haven’t smoked in weeks. Those eleven states include Arizona, Delaware, Georgia, Illinois, Indiana, Iowa, Michigan, Oklahoma, Rhode Island, Utah and Wisconsin.

These states have no-tolerance laws for driving under the influence of any drugs, but marijuana differs from other drugs in that THC—the intoxicating compound in marijuana, is metabolized in the fat cells, which means it stays in the body much longer than other illegal and prescription drugs. It also means that people who are regular users of marijuana run the risk of being charged with a DUI even after the intoxicating effects have long worn off. These states are essentially punishing a person for their past behaviors.

At least 12 states, including Colorado and Washington have “per se” marijuana DUI laws. This means a driver can be charged with marijuana DUI if THC levels in their blood exceed the state’s legal limit, which is 5 Nano grams per milliliter of blood. Chronic medical and recreational marijuana users can easily be over the legal limit long after the effects of THC have worn off. In Colorado, unlike Washington, a person charged with marijuana DUI can present evidence to show that they were not under the influence of the THC when they were charged.

The majority of states have effect-based laws, meaning that law enforcement must be able to prove that the driver is directly under the influence of marijuana before they can be charged with driving under the influence.

A map available here shows  marijuana DUI laws by the state. If you are a medical or recreational marijuana user, it would be wise to learn the drugged driving laws in your state.

The laws against drugged driving are in place to protect everyone, but the marijuana activists say that “per se” and “no tolerance” laws lead to unnecessary and unjust DUI charges. If you have the misfortune of being charged with marijuana DUI, a DUI attorney can help you build an effective defense.