DENVER — People who use alcohol and marijuana together may be at greater risk for alcohol-related problems, such as drunken driving and poorer health, than those who use only alcohol, a new study finds.

In the study, researchers analyzed information from people in Washington state who were asked about their use of alcohol and marijuana over the past year, and whether they had ever experienced problems from their drinking. The survey took place from 2014 to 2015; recreational marijuana use was legalized in Washington in 2012.

Of the more than 2,400 people who said they drank alcohol in the past year, 70 percent said they used alcohol only, 18 percent said they tended to use alcohol and marijuana simultaneously, and 13 percent said they used both drugs, but separately.

Those who used both drugs simultaneously reported drinking more frequently, and consuming higher amounts of alcohol, than those who said they used both substances separately, as well as those who used only alcohol, the researchers said.

What's more, the people who simultaneously used alcohol and marijuana were at a greater risk of experiencing problems from their alcohol use, compared with those who used only alcohol. Simultaneous users were three times more likely to drive drunk, 6.5 times more likely to experience alcohol-related financial problems and four times more likely to experience alcohol-related health problems, compared with those who used only alcohol, according to the study. The study was presented here on Monday at the meeting of the American Public Health Association. 

The findings suggest that in order to minimize harms from alcohol "people who use both [marijuana and alcohol] should probably use them separately," said study researcher Meenakshi Subbaraman, a biostatistician at the Alcohol Research Group, part of the nonprofit Public Health Institute in Emeryville, California. And in states where marijuana is legal, policymakers might consider requiring warning labels on marijuana products about the risks of combining the substance with alcohol, Subbaraman said.

People who used alcohol and marijuana separately were not at increased risk of alcohol-related problems compared to those who used only alcohol.

The results are similar to those of a 2015 study conducted by the same group of researchers. That study, which surveyed Americans in all 50 states, also found that those who used alcohol and marijuana together were at greater risk for harms, such as financial and health problems, than those who used alcohol only.

However, the 2015 study also found that people who used the two substances simultaneously were at greater risk for drunken driving than those who used the two substances separately. The current study did not find a difference between drunken driving risks for the simultaneous users versus those who used the two substances separately, which was surprising, Subbaraman said. It could be that the current study did not have enough participants to detect a difference, and so future research is needed to look at this question, Subbaraman said.

The current study was also conducted at a single point in time, so it cannot prove that using alcohol and marijuana together is actually the cause of the increased risk of problems. It could be that those who use the two drugs together are more impulsive, or that they are more likely to use the drugs in certain locations (such as at parties versus at home), which could account for the increased risks, Subbaraman said. More research is needed that follows these people over time to get a better understanding of the reasons behind the link.

Original article on Live Science.