A few weeks ago, The Wall Street Journal ran an op-ed about the need for a reliable DUI test for marijuana. At the moment, the only way to know if a driver has been using marijuana is if they confess. The authors asked for more resources to be put into creating a test for marijuana DUIs.

However, The Wall Street Journal released another op-ed yesterday in which an expert questioned whether such a marijuana DUI test is even possible. Robert L. DuPont, president of the Institute for Behavior and Health, Inc., says that unlike alcohol and other drugs, there's no magic level for marijuana impairment in drivers. Scientists have been unable to determine what level of marijuana detected in tissues is enough to be considered "impaired." 

There's also an issue because of the different forms of marijuana. Smoking cannabis versus edible products have different metabolism rates, meaning the body breaks down THC at different speeds depending on how the user ingests it. 

Now, DuPont is a somewhat biased source. He previously served as the Drug Czar during the Nixon and Ford administrations and has referred to marijuana as "the most dangerous drug." However, his opinions on marijuana and DUI impairment are shared by even pro-marijuana advocates. In July, NPR ran a story about the struggle faced by scientists trying to find a reliable DUI test. While there are some hints of breakthroughs on the way, even the optimistic are unsure if there will be a marijuana DUI test in the near future.