assume you mean chronic, regular use of large quantities of marijuana. if so, the answer is *kind of* no, but there’s a big asterisk and ifs and buts involved. here’s what most certainly can and does happen to most chronic users:

1. lose some functional short term memory (this issue stays for as long as you continue to smoke, the minute you quit it’s gone) as you forget where you kept your keys, why you came to the kitchen, small tasks that need to be done, uninteresting appointments and work that needs to be completed, etc.
2. suffer cognitive deficits, like the ability to focus on tasks that you may not want to do. everyone has these in their life, whether it’s filing taxes or going and getting something boring done. very hard to get off your ass and do them after regular chronic use.
3. face shortfalls in attention span. things that are not interesting enough and exciting enough will not keep your attention, and your thoughts will begin to drift, almost uncontrollably, to the point where it feels like it’s raining ideas and thoughts in your head, and all you have is a tiny umbrella.
4. the inability to manage time effectively. being regularly late is common among stoners, unless they’re going to refill their stash jar.
5. slightly slower processing for mental skills that you don’t use often. in today’s world, that is like mental calculations, learning new complex concepts, etc.

now, this is not becoming dumber. but, meet someone who forgets where they keep their keys and wallet all the time, can’t focus on important work at hand, has a short attention span, is always late for important tasks, can’t plan for nuts, and takes time or requires triggers to remember things that have been told to them a few times already, and it’s easy to see why one might consider a chronic marijuana smoker ‘dumb’ when what’s actually happening is that his/her intelligence is not directly affected but there are definite cognitive deficits that are experienced that impair the expression and use of that intellect.