I knew a guy that told me that he used to smoke marijuana daily for many years. Although he enjoyed it very much, he went on to tell me that after some time, he quit. His abstinance so he says, was about 5 years. He mentioned that when he tried to start up again, he had massive panic attacks. He explained to me that there was no positive affect, only terrible fear that was the worst thing he ever experienced. He even mentioned that he tried some marijuana from different sources and at different doses for the next couple of months yet he had terrible reactions every single time. So given that, he told me that he decided to quit (this was a couple of years ago) and he hasn't gone back since. I wonder what changed with him to make him react in such a way to something that he previously enjoyed so much. Any ideas?