Jun 24, 2015 10:47 AM EDT By Staff Reporter, UniversityHerald Reporter

Marijuana could dangerously impact the way you drive, according to a recent study.

Researchers at the University of Iowa found that drivers who use alcohol and marijuana together weave more on a roadway than drivers who use either substance independently. However, the cocktail of alcohol and marijuana does not double the effect of the impairment.

"What we saw was an additive effect, not a synergistic effect, when we put them together," Tim Brown, a corresponding author of the study, said in a statement. "You get what you expect if you take alcohol and cannabis and merge them together."

For the study, researchers recruited 18 people between the ages of 21 and 37 years old who reported drinking alcohol and using marijuana no more than three times a week. They were given 10 minutes to drink a mixed drink with alcohol or plain juice in an alcohol-rimmed glass and topped with alcohol to mimic alcohol taste and odor. The idea was to get the participants blood alcohol level to about .065 percent at the start of the simulated drive.

Next, they were given 10 minutes to inhale a placebo or vaporized cannabis using a vaporizing system designed in Germany called "Volcano Medic."

They found that participants who consumed only alcohol weaved more during a 35- to 45-minute simulated driving test than those who consumed only vaporized cannabis.

In 2014,  the National Highway Traffic Safety Administration's Roadside Survey of Alcohol and Drug Use by Drivers found the number of drivers with alcohol in their system has declined by nearly one-third since 2007. However, that the number of drivers with marijuana in their system grew by nearly 50 percent.

To date, medical marijuana is legal in 23 states and the District of Columbia while marijuana has been approved for recreational use in four states and Washington D.C.

The findings, which are detailed in the journal Drug and Alcohol Dependence, could help shape future legislation in the United States where some experts say policies on drugged driving are woefully behind.

See Now: Covert Team Inside Newsweek Revealed as Key Players in False Human Trafficking Lawsuit