As marijuana legalization builds momentum across the United States — with Michigan becoming the latest state to allow recreational use by adults — researchers are warning that more studies are needed on the long-term effects of chronic pot smoking on the human brain, NBC News reports.

Marijuana is the most commonly used illicit drug in the United States, but little is known about its effect on health or how addictive it is.

According to a 2017 poll conducted by Marist College and Yahoo News, more than half of American adults have tried marijuana at least once in their lives, and nearly 55 million of them, or 22 percent, say they use it currently. Close to 35 million are what the survey calls "regular users," people who say they use marijuana at least once or twice a month.

"Surprisingly, many people freely admit to using marijuana, but underreporting remains an issue," said Jonathan Caulkins, a drug policy researcher and professor at Carnegie Mellon University. "To correct for that one should fudge upwards by a factor of 20 to 40 percent."

With Michigan's Election Day ballot measure, 10 states and the District of Columbia now allow the drug's open use; 33 states plus D.C. allow medical use, leaving many to wonder if the U.S. will follow Canada's lead in legalizing marijuana nationwide.

Studies have shown that chronic marijuana use affects the same brain structures that are involved with addiction.

Marijuana use disorders are often associated with dependence — in which a person feels withdrawal symptoms when not taking the drug. Frequent users report irritability, mood and sleep difficulties, decreased appetite, cravings, restlessness and physical discomfort that peak within the first week after quitting and last up to two weeks. Marijuana dependence occurs when the brain adapts to large amounts of the drug, requiring more and more to create the desired euphoric effect.