Well, I'm just gonna say arguing anecdotally is pointless. However, functional opiate and amphetamine users are not "hiding a big problem". In many cases, this is enabling them to deal with high-stress situations, or think more clearly. One example is Paul Erdős, a brilliant mathematician that used amphetamines quite a bit and found no reason to not use them. No hiding, no problem, no "keeping up appearances".
Don't by into the "addiction is bad" story that DARE and the like sell.