THC (Tetrahydrocannabinol) activates cannabinol receptors in one's brain whenever they smoke cannabis. These receptors are responsible to stimulating hunger (why you get munchies) but they also stimulate something called long term depression, which is the process by which your brain forgets memories. The effects take years to decades for anything significant enough to precipitate.

Now, I'm not against smoking cannabis just like I'm not against drinking alcohol. I'm for people understanding the risk factors and being fully informed before they make a decision.