The Scientist: Matthew J. Smith, Ph.D., research assistant professor at Northwestern University Feinberg School of MedicineThe Answer: Surely there’s no lack of anecdotal evidence that smoking weed impairs memory. (Just ask anyone who’s ever asked a high friend to bring them back some munchies from the convenience store.) Multiple scientific studies support the observation that people have problems remembering things when they are actively under the influence of cannabis. Other research suggests that the effects last long after you’ve sobered up—and even long after you quit smoking all together.Smith and his colleagues published a study in 2013, which found that young adults who smoked marijuana daily as teenagers showed differences in the shape of their brains and performed worse on memory tests than non-smokers. The younger they started, the bigger the effect. And this was after two full years of not smoking pot. Whether or not these changes are reversible is unclear. While Smith’s research found that the effects can linger for years, other studies have shown that brain function can return to normal after three or four puff-free weeks.There’s not much research into how much or how often you have to smoke for there to be a real or lasting effect, but some data suggest that even the occasional toke takes its toll on the brain. How this translates to memory or behavior, however, is a yet unanswered question in the field of pot studies.MORE: Hypnosis for Quitting Smoking (And Much More)