Marijuana can cause psychiatric problems because it throws off the balance of neurotransmitters in the brain.
I believe it.
I've discussed this with people for years, and what I keep hearing is that there is no long-term damage, that the THC merely locks onto receptors in the brain harmlessly, but it's always been difficult to me to dismiss the possibility that neural pathways are altered by longterm exposure to THC. I strongly believe that pot use wreaks havoc on the limbic system and seriously limits a person's ability to problem-solve and change behaviors at their basest emotional level.
If you have to have pot or booze to make it through every single day, you need to change something in your life. These things don't fix what's wrong. Why squander life by anaesthetizing oneself to experience? Only you can change the way you live.