The study recruited 18 occasional cannabis smokers -- 13 of them men between 21 and 37 years old. The participants took six 45-minute drives in the National Advanced Driving Simulator, the most sophisticated driving simulator of its kind, and a different combination of high or low concentration THC, alcohol and placebos were tested in each drive.
The study found that combining cannabis with alcohol impairs drivers more than consuming just one or the other.
"Driving is a very complex task and this is why cannabis, both alone and with alcohol, is so dangerous to drive," Huestis said.
Researchers also revealed the first comprehensive analysis of medical marijuana's potential benefits. Researchers found medical marijuana can help chronic pain and muscle stiffness in multiple sclerosis, but there is weak evidence it helps with other conditions like anxiety, sleep disorders and Tourette's syndrome.
Edible products are also under scrutiny. Researchers evaluated nearly 50 brands of medical marijuana products, including candy, baked goods and drinks bought at dispensaries in Los Angeles, and found a majority had inaccurate amounts of THC listed on many products.
Nearly one in four had higher amounts of THC than labeled, which could cause ill effects, but most had lower-than-listed amounts, meaning users might get no effect.
Researchers behind most of the studies say more analysis needs to be done. On Monday, the Department of Health and Human Services made it a little easier for privately funded medical marijuana research to get approved. Researchers will no longer need to submit proposed marijuana studies to the U.S. Public Health Service for review.