Epidemic is a tricky concept given how much it gets throw around in popular discourse. I did research in communities where it was assumed that a methamphetamine epidemic was underway and there was little discussion of crack even though all qualitative and quantitative data indicated that crack was far more commonly used. I did research in another area where Meth was portrayed as the big deal although there were far more deaths from opiate overdose. Issues like race and geography can come into play as well.

You might take a look at the literature on moral panic as it pertains to drug use. Sometimes there are very specific reasons why people want to emphasize use of a particular drug as an epidemic.