Like at many campuses across the country, the drug scene depends on with whom you associate. While the most visible drug use on campus is definitely cigarettes, harder substances are also abused, but you won't necessarily see them. On the other hand, at a large number of events, you'll see students having a great time being sober, without the use of any alcohol or drugs at all. All in all, though, whether you are interested in partaking or not, it's totally up to you. You can go all four years without seeing drugs, or you can engage in some heavy activities.
Since CSUN is a commuter school, the University is not as conducive for a drug or "party" environment as other schools, since there isn't a strong residential community. Sure, in the dorms, you'll occasionally smell marijuana wafting from a room, but it's not as prominent as at other universities. However, those that do partake in alcohol or drugs in the dorms need to be discreet, as police enforcement is pretty strong.