The chief ingredient in cannabis is THC, or tetrahydrocannabinol. Contrary to popular belief, THC is a cannabinoid that very closely resembles cannabinoids that are already circulating in our body. These cannabinoids circulate throughout us at a much lower concentration, however, as opposed to the large quantities during the intake of, say, marijuana smoke. Before moving on it is imperative to take a small detour and explain, in part, how your brain functions normally. Throughout your brain there are millions upon millions of neurons that are composed of, mainly, an axon and a dendrite. These neurons communicate by sending neurotransmitters from the axon of one neuron to the dendrite of another. These interactions happens thousands and thousands of times per millisecond -- every time you blink, think, breathe, move, etc. It's the fundamental process of how your brain makes things happen. However, there is a short refractory period experienced by each neuron immediately after it has passed on any neurotransmitters. This prevents the neurons from being overworked, so to speak. Back to the cannabinoids. When you inhale marijuana smoke the large influx of cannabinoids directly affect your neurons and how they function, namely in the sense that they temporarily eliminate this refractory period, allowing your neurons to put in some overtime. This often leads to one idea taking up your entire thought process, making it seems incredible and profound regardless of what this thought is. It could be plastic forks. Seriously. Most times this prevents you from seeing the bigger picture, or even recalling your last epiphany, because you become so caught up in the momentum of your current idea. These cannabinoids also affect the levels of dopamine and nor-epinephrine in your brain, often causing feelings of euphoria, relaxation, pain modulation, and general enhancement of an experience. Moreover, there are cannabinoids in areas of your brain that govern short term memory and learning, coordination, movement control, and higher cognitive function. tl;dr -- no, it doesn't rot your brain.
