What is the Legal Limit of THC in Blood?

Learn about what is considered legal limits for THC in blood and how it affects drivers from an expert's perspective.

What is the Legal Limit of THC in Blood?

The government has proposed that a minor violation applies to drivers who test between two and five nanograms of THC per milliliter of blood. Drivers who test above five nanograms, or who have a blood alcohol concentration greater than 50 milligrams per 100 ml combined with a THC level of 2.5 nanograms, would receive current penalties under the Penal Code for drunk driving. To address driving under the influence of marijuana, each state has a law that, in some way, addresses the issue. Some states have ongoing or active oral fluid analysis programs on the road to detect the presence of marijuana and other drugs that could affect driving behavior.

In addition, states are exploring methods to improve legislation and enforcement of the Drug Driving Act (DUID) by better collecting data on accidents and citations. After the legalization of marijuana in the country, the number of drivers with moderate injuries in car accidents who exceeded the legal THC limit more than doubled, from less than 4 percent to 8.6 percent. Studies agree that the combination of alcohol and THC is particularly dangerous, if anything worse than “drunk driving”. The government has proposed a legal shortcut that allows police to file a complaint for driving under the influence of alcohol based on the driver's blood level of THC, without having to further demonstrate their disability.

Road tests could be improving with the arrival of a saliva test that could provide more immediate and reliable information on the level of active THC in a person's body. Unlike alcohol, which is metabolized at a relatively constant rate, THC is metabolized at an exponentially decreasing rate, where the level of THC in the blood first drops rapidly, followed by a slower decline as lower levels of THC are reached. Chronic users who develop tolerance to THC can, in some cases, drive safely with very high levels of THC in their blood. Unlike the blood alcohol level of 0.08, which is widely accepted as indicative of drunk driving, establishing a credible level of THC has been difficult to achieve.

A person may be affected by the use of marijuana with blood THC concentrations below the detection limit of the method. Rare marijuana users can pass a blood test since the THC in their body dissipates quickly; however, frequent marijuana users can still be charged for operating while intoxicated (OWI) even though they haven't used marijuana in weeks. The NHTSA recognized last month these gray areas in a report to Congress that not only questioned the reliability of tests for detecting THC but also pointed to the problem of determining whether a driver is too high to be behind the wheel. It is not advisable to try to predict effects based solely on blood THC concentrations, and it is currently impossible to predict specific effects based on THC-COOH concentrations.