Well, I smoke a lot of weed and so far didn't have any problems with it.
I know that it isn't harmless, but it's uses far overcome the problems it brings.
Used the right way (Vaporizer etc.)it heals physical as well as psychic problems,helps in a great number of eye-related problems and is used against chronical pain.And it get's you high in a completely controllable way depending you know yourself and how a different amount and different ways to consume it react with your body.