Drivers who use medical marijuana are posing a challenge to law enforcement officers trying to enforce driving under the influence (DUI) laws, according to the Associated Press.

Unlike alcohol, the active ingredient in marijuana, THC, can stay in the system for weeks, and there is no easy test to determine a person’s level of impairment due to the drug, the AP notes.

A recent analysis of studies found driving under the influence of marijuana is associated with an increased risk of a motor vehicle crash, especially for fatal collisions. The analysis found driving under the influence of marijuana was associated with almost twice the risk of a motor vehicle crash compared with unimpaired driving. The studies in the analysis included nearly 50,000 people.

A 2009 report from the National Highway Traffic Safety Administration, based on blood, breath and saliva tests collected on weekends from drivers in 300 locations nationally, found that 16.3 percent of drivers at night were impaired from legal or illegal drugs, including 9 percent of drivers who had detectable traces of marijuana in their system.

Several states have set limits on the amount of THC that drivers can legally have in their blood. But there is no widespread agreement about what that limit should be. While two states have set a limit of 2 nanograms per milliliter of blood, other states do not allow any THC. Colorado and Washington state are debating a threshold of 5 nanograms, according to the article.

Several factors can influence THC blood tests, including weight, age, gender, and frequency of marijuana use.