Muffin: That’s really a very good reason for avoiding drugs. Much better than the rather insulting “I don’t want to mess up my life,” which implies that, because I do drugs, my life therefore must be messed up.
When I was a kid I was a finicky eater (see George Carlin’s monologue on the subject). If I don’t like the taste of something, then I don’t ingest it. As it happens, I don’t like the taste of alcohol. Some of my favorite desserts are flavored with rum or brandy, but IMHO most would taste better without the booze.
I’m allergic to tobacco. I’ve been told that I turn green when I sit in the smoking section of a restaurant. I never felt any urge to smoke or chew.
I’ve been to a few dentists who used laughing gas. It’s mildly amusing, but on the whole, I prefer sobriety.
I have never in my life had a headache bad enough to warrant an aspirin. (OK, not completely true. After cranial surgery, the doctors prescribed some Tylenol-2. However, they gave me a two-week supply, and I stopped taking them after four days, so I still think I can brag.)
As for recreational drug use, I never saw much point in it. I’m happy enough sober. I don’t see any need for artificial augmentation. Once, in college, I realized something: I had seen my friends sober; I had seen my friends drunk; and I liked them better when they were sober.
Besides, addiction is rather undignified, and I am unwilling to take the risk.
Everyone is certainly entitled to make their own choices at to what they do or do not ingest.
A couple of points I wanted to address though.
Several posters indicate that (at least a part of) the reason they did not try/do drugs/ETOH is because they “heard” xyz. Depending on who you are talking to, you can hear all kinds of things. Not all of which are accurate. But if you are content in basing your actions on hearsay, that is your choice.
Several other folk comment along the lines that they have “better things to do.” IME, drinking or using many if not most drugs in moderation one evening, will have little or no effect on your ability to function the following day, not to mention the following week. And I would be suspicious if a great number of folk alleged that their lives were so darn full and productive and their systems do delicate that they couldn’t lose the couple of hours of peak performance that would be lost by sharing a joint or having a drink a couple of hours before going to bed. Most people I know (users and non alike) find some ways to waste time, whether sober or not.
Now if your concern is that you will not have the self control to keep your usage in moderation, that is a valid concern. I guess the recognition of a potential weakness can be viewed as a strength.
Just don’t believe everything you may have “heard” about how dangerous, unpleasant, powerful, and habit forming all substance usage is. Make whatever choice you feel is best for yourself. And no one else should presume to tell you that it is not the correct choice for yourself. Of course, if you have not at least taken that initial sip/puff/etc. to find you did not enjoy the activity or its implications, you should be honest enough with yourself to acknowledge that to some extent your choice is being made out of ignorance.
You can add me to the ‘never taken drugs’ club!-
I have never taken drugs because I never got the opportunity to try them, and it has to be said, even if I did, I probably wouldn’t have. Call me Cautious Square Lil if you like but I kinda like being alive-there are so many ways to kill yourself why actively court another? (note:this should not be construed as an attack on people who make the personal choice to take drugs-I respect everyone’s choice and this is mine)
But anyway I was always hearing things like “every teenager in Britain knows where to buy cannabis and LSD” and I was like "hey! I don’t know where to get them! Being horribly shy and having no friends will do that to you I guess 
I had a romantic prospect (who turned out to be married already and I knew because I found his website that said so! How stupid did he think I was?) but anyway he regaled me with tales of how he took LSD and I was so jealous! This might sound odd considering what I have already said but I kind of think that having an LSD trip is one of those experiences that everyone should have before they die, and I really really wanted to experience it. But I am a bit prone to depression and people who are prone to that should probably avoid it, and hey, I have no idea where to get it anyway! (Being horribly shy and having no social life will do that to you! 
)
(oh! and at the age of 29 I have no idea what the apparantly distinctive smell of cannabis is like! Yes I have had the most sheltered life of any person alive. I know! )