The effects of marijuana are quite variable. The quality control on it is not very good and sometimes it may contain other chemicals. Marijuana can sometimes cause/trigger anxiety attacks. Occaisionally, people have them for a while (weeks) after stopping marijuana. Serious psychological derangement after marijuana use is pretty rare. The context in which it is used is a heavy determinant of the experience and having changes in sensation from a psychoactive drug in less than secure surroundings can more often trigger anxiety.