A study conducted by the National Institute on Drug Abuse found using marijuana and alcohol together impacts driving more than using either substance alone, Time reports.

The study included 18 occasional marijuana smokers—13 of them men—between the ages of 21 and 37. They took six 45-minute drives in a driving simulator.

Before using the simulator, participants consumed specific combinations of marijuana and alcohol, or a placebo. They used a vaporizer to consume marijuana because the study took place at the University of Iowa, which is smoke-free. The study participants gave blood and saliva samples so researchers could verify their intoxication levels. The researchers measured their levels of THC (the psychoactive chemical in marijuana) and their blood alcohol concentrations.

The researchers looked at weaving within the lane, the number of times the car left the lane, and the speed of the driver’s weaving. They found alcohol had an effect on the number of times the car left the lane and the speed of weaving, while marijuana did not. Marijuana use did increase weaving, the study found.

Drivers who had a blood concentration of 13.1 ug/L THC (more than twice the legal limit in Washington state and Colorado) showed an increase in weaving similar to those with a .08 breath alcohol concentration, the legal limit in most states.

Drivers who used both alcohol and marijuana weaved within lanes even if their THC and blood alcohol concentrations were below the threshold for impairment taken on their own. Lead researcher Dr. Marilyn Huestis told Time, “ We know cannabis is primarily found with a low dose of alcohol. Many young people have a couple beers and then cannabis.”

The new study looked at the concentration of THC in the driver’s system while they were driving, Huestis noted. When a drugged driver is on the road, their blood may not be checked until several hours after they are arrested, allowing their THC level to drop, she said.