At a minimum, I think it causes people to lose focus and motivation. I had friends in college, and myself, who changed majors or dropped out due to missing class or falling behind in studies. All of us daily pot users. I'd also argue it causes anxiety and depression for a lot of people. But these are harder things to prove than the direct physiological impact of alcohol on your liver, for example, so a lot of people are skeptical. Pot's more insidious.