Marijuana is now more accessible — legally — than it has been since it was first banned in the "Reefer Madness" era of the 1930s, but that doesn't mean researchers think we fully understand the plant or how its use affects people.

Far from it.

We know enough to say that marijuana has some legitimate medical uses and to say that in many ways, it's less likely to harm users than substances like alcohol or opioids, but researchers still have a long list of questions.

Government regulations make the plant extremely difficult to study, which is one of the main reasons there are still so many things to learn about marijuana.

Business Insider recently spoke to several prominent researchers to see what they think the most important questions are — and what's being done to answer them.

Here's what they hope to find out.