Delta-9-tetrahydrocannabinol, or THC is the active ingredient in cannabis. This chemical interacts with a wide range of systems, giving rise to an equally wide range of effects. The “high” is therefore a pretty diverse one, with users reporting everything from euphoria and calm to paranoia and anxiety. When smoking marijuana the THC passes from the lungs to the bloodstream, then it's picked up by two receptors; cannabinoid receptor (CB) 1 and 2. These long, rope-like proteins weave themselves onto the surfaces of cells all over the body, which helps explain increased heart rate, red eyes, pain relief and other effects that are not purely psychological. Most of the action takes place in the central nervous system, where THC is carried by CB1; the compound typically has about two to four hours to toy with a range of neurological functions including, memory formation, appetite, and time perception.
In my opinion Marijuana is a personal choice. If people choose to smoke it, so be it. It's their lives and if they want to enjoy one of earths beautiful creations they should.