I guess those are my three big problems with drug use, namely being that they stop people from thinking at their best (it's difficult to perform any sort of task when your busy thinking that the walls are all alive), they are extremely addictive (you thought quitting coffee was hard? Try quitting a hard drug and it's many times worse), and the third is that they ruin your health and your relationships with those you know.
Always good to remember that there are plenty of illegal drugs that will safely do none of these things, though. In fact, many do the exact opposite - some of the most abused drugs in the US are drugs specifically geared towards allowing people to focus better and think at "better than their best". Oddly enough, LSD is of one of the few drugs clinically proven to literally push even the best minds significantly past their limits - there are a number of breakthrough theoretical and technical discoveries due to the studies into LSD (before the government banned it). In fact, research indicates it is the single most capable drug at increasing people's ability to solve creative and technical problems, and it does it exceptionally well.
If you're interested in learning more, the Institute for Psychedelic Research at San Francisco State College spearheaded most of the more valuable research.
Well, I don't want to do drugs and I don't want to associate with people who do drugs, so my life plan is to not have them figure into my life plan. In this case I think it's best to assume that they're bad and then not talk about them in actual social situations.
And finally, I'm sorry - but this makes you kind of a huge asshole. It's like saying "I don't want to associate with people who don't believe in my god" or "I don't want to associate with people that have ever voted for a non-democrat" or "I don't want to associate with people that have ever had sex" because, "It's best to assume those things are bad". It's certainly your right not to do so, but it still makes a huge dick.
Also, probably a hypocrite, because I'm almost certain you've done drugs, will continue to drugs, and will likely end up doing some rather heavy drugs sooner or later at some point in your life. So it isn't even doing drugs, it's ostracizing people for doing drugs for reasons you don't agree with.
(Unless you're way more dedicated to this than I give you credit for, and you avoid caffeine, alcohol, painkillers and antibiotics, psych meds, as well.)
And if you plan on disassociating with those drug-takers as well, uh, you're probably going to lead a pretty lonely life. Especially since the most often abused drugs ARE prescribed drugs.