Mediator or Moderator? I am attempting to ascertain whether drug use is a moderator or a mediator in the following scenario.
There is evidence to suggest neuroticism impacts academic performance and drug use may help explain this association. Would drug use be considered the moderator or mediator in this example?
 A: You have to be a bit more specific in your language.

drug use may help explain this association

Do you mean that there is a correlation between the 3 variables in the population, but the correlation between neuroticism and performance disappears or changes when you condition on drug use ? If that is what you mean then it is consistent with drug use being a mediator.
Moderation is a different thing. Moderation occurs when a variables changes the association betwen two other variables. So, suppose there is an association between neuroticism and performance, which is larger (or smaller) for higher levels of drug use. In a regression model, we would find a meaningful interaction between drug use and neuroticism if this was the case.
It is, of course also possible that mediation and moderation both occur.
In your particular case there is also another possibility: If you think that drug use affects neuroticism, and drug use also affects performance, then, if neuroticism also affects academic performance either directly or indirectly, then drug use is a confounder of the association between neuroticism and academic performance.
Again it also possible that confounding and moderation both occur.
