With 23 states currently legalizing medical cannabis and four states permitting marijuana for recreational use, driving while high is becoming as much of a concern as driving drunk for some legislators. However, a new study has found that mixing the two substances can be particularly hazardous because it greatly increases the amount of THC in the bloodstream.

The findings, reported on in Science Daily, came from data in a controlled study of 19 cannabis users. All of the participants inhaled vaporized cannabis, but one group consumed a low dosage of alcohol 10 minutes before smoking and the other group consumed a placebo.

Researchers found the alcohol-consuming group had a much higher blood concentration of THC than the placebo group, leading them to conclude that a motor vehicle accident was more likely when consuming both substances instead of just one.

“The significantly higher blood THC ... values with alcohol possibly explain increased performance impairment observed from cannabis-alcohol combinations,” said lead researcher Dr. Marilyn A. Huestis.

A separate study published last April also found that those who smoke weed while drinking were far more likely to drive drunk than those who only consume alcohol. The Alcohol Research Group, a program of the Public Health Institute, analyzed data from over 9,000 participants in two national surveys conducted in 2005 and 2010. Approximately 7.5% of the participants combined marijuana with alcohol, while 4% used them separately and 89% only drank.

The scientists found those who consumed both substances were twice as likely to drive drunk than one-substance consumers. They were also six times more likely to binge drink at least monthly, and three times more likely to have social consequences surrounding their use: physical fights, lost career opportunities, and brushes with the law.

The U.S. Centers for Disease Control and Prevention report that marijuana and cocaine are involved in about 18% of auto-related driver deaths. Alcohol was often found in combination with these drugs during later autopsies.