There's a lot of work being done on the concurrent use of alcohol and drugs, particularly THC, because it's often prevalent. The question is the impairing effect. Is it 1 plus 1? Is it 1 minus 0.5? Is it 1 plus 2? The effects of alcohol are quite different from the effects of THC on the body. Alcohol typically affects the back region of the brain; marijuana typically affects the top section, the cognitive functions. There are some antagonistic effects, and there are some complementary effects. This is not well known, and there's a lot of research yet—
On September 27th, 2017. See this statement in context.