As the movement to authorize cannabis in the United States gains momentum, scientists worry about keeping the public safe, particularly on the roads. Recent studies in which cannabis users took controlled doses of cannabis in the lab have recognized new biomarkers that can be used to estimate a person’s recent cannabinoid intake. But, using those markers to judge cognitive and behavioral impairment is complex.

Alcohol can damage a user more than cannabis, and indeed, the risk of an accident while driving upsurges in proportion with blood alcohol concentrations. But pot is different: many variables can affect how impaired someone is at any given concentration of tetrahydrocannabinol (THC), the primary psychoactive agent in cannabinoids. Whether it is inhaled or consumed, or whether the user titrates their own dose, can affect the level of impairment. And pairing marijuana with alcohol makes the high higher, and the alcohol buzz last longer.

Another problem is that THC quickly leaves the bloodstream. Previous research has shown that while an infrequent user is impaired for 6 to 8 hours, blood THC concentrations can be effectively zero after 2.5 hours. And on average in the United States, it takes from 1.4-4 hours after a crash or traffic stop to run a blood test.

Long-term daily marijuana users, like those who use marijuana for medical reasons, also present a challenge for developing roadside protocols. THC amasses in the tissues of the body and then slowly releases over time, meaning that long-lasting users can test positive for cannabis even after 30 days of abstinence. Psychomotor impairment can be observed three weeks after the last dose.

The implications go beyond driving. These new markers and tests could also be used to assist in treating drug dependence, in determining suitable therapeutic levels of medical cannabis, and for monitoring women who want to stop using cannabinoids during pregnancy.