Certainly the gateway aspect along with the effects of smoking or ingesting mariuana itself can cause vehicular accidents and damage to oneself and others at home and work. But the physical biological dangers are quite profound as well.
When marijuana is smoked or eaten, THC enters the lungs and then makes its way into the bloodstream. The blood drives this harmful chemical to the brain and throughout the organs.
THC, weed addiction, can cause brain damaging effects; and certainly in younger users, it affects brain development. Some researchers believe that long term usage can cause permanent irreversible damage.