I used to blame past intelligence deficits or periods of cognitive slowness on weed. Not anymore, now that I have more experience with how I react to substances.
I have smoked/vaporized a half pound of marijuana since August, and I am doing better than ever before in my life. I have managed to stay happy for most of this time, a true feat for my typically depressed self. I hold an intense job that requires a whole lot of responsibility, dedication, interpersonal skills, creativity, common sense, organization skills, technical knowledge and quick thinking. I am learning a 2nd language in my spare time and I read literature incessantly. Also I program techno music and play guitar. I'm pretty much constantly motivated apart for the odd short, harmful bursts of sexual energy and anger that weed helps to quell.
I have not been using any drugs other than marijuana (in great excess) over this great period of my life. I have come to realize that hard drugs damage my brain temporarily, and that alcohol use is the worst culprit of all. Even having a few drinks will drastically decrease my awareness and productivity the next day, but weed only ever enhances my consciousness and doesn't come with a down (unless I stay off it for more than a day or 2, in which case I start hating on life because I'm addicted to weed). I'm feeling smarter than I've ever felt in my life, and I owe this to my current dietary habits, and the complete absence of drugs other than weed and tea especially that dirty alcohol shit which causes a rediculous amount of brain and other organ damage in my experience.
It's easy to point the finger at cannabis if you're using it daily, but maybe if you're feeling dumbed down at this time in your life, then it is because you are allowing your motivation to slide, or because you are using other drugs than pot that are actually neurotoxic. Alcohol causes brain damage; weed creates changes in the brain but in my experience these changes have been mainly positive apart from the addiction and fiending, which isn't always so bad.