Unless you used to have a serious meth or heroine problem I'd say you're fine. ;)
I was pretty honest about my drug use. The way they phrase the questions sometimes leaves you with a bit of doubt, though. Like, "How many times did you use this drug?" I'm not sure who keeps a really diligent record of how many times they do drugs at parties.
But, yeah, I tried to be as accurate as possible, modulo that doubt about how possible it is to accurately answer these questions.