It surely lowers you IQ, although it still may still be a net positive. Many brilliant people are also anxious, and can't get things done because of it. If marijuana helps you relax enough to focus, this is a win. I don't smoke, and I've hardly done any sorts of drugs at all ever actually, but I kind of get the appeal when I take Benadryl (I know it's weird), because the tiredness relaxes me and I stop worrying about a lot of peripheral problems.