Quite a few smart people have trouble getting past the idea that recreational drug use is going to permanently damage their brain -- they're often worried that they'll end up losing ten or fifteen of their precious IQ points. Below are my justifications for limited and controlled drug consumption. Bear in mind that they aren't intended to justify abusing potentially damaging drugs; use does not equal abuse. If you roll every weekend for a year, drink scores of bottles of DXM, or inhale nasty stuff like keyboard cleaner, you're going to suffer noticeable and permanent damage. If you accept the trade of mental ability in the future for a good time now, then that's your prerogative, but my arguments break down for such heavy usage.

All of that said, a lot of drugs have gotten a bad reputation even though they cause little or no damage at all! LSD, psilocybin, and the rest of the classical psychedelics do exactly zero brain damage, and marijuana doesn't do any either. Dopaminergic drugs (coke, speed, methamphetamine) tend to do only a little bit, either by overworking transmitter neurons or undersensitizing receptor neurons, but when used in moderation this shouldn't be permanent. Rolling on ecstasy is relatively bad for your serotonin neurons, but doesn't appear to adversely effect any other brain areas -- of course, ruining your serotonin subsystem may make you an emotional wreck later in life, so heavy MDMA use is a bad idea. Alcohol and DXM are both notoriously neurotoxic and should probably be avoided as much as possible, but limited use (ie, DXM once or twice a year, getting drunk every couple of months) probably won't cause enough damage to be noticeable.

Anyway, here is my reasoning:

1) Human brains have deep redundancy. Thousands of neurons die naturally every day, so the brain has an innate ability to withstand damage without loss of function. Each concept or memory is partially represented in probably tens of thousands of neurons in whatever specific locus on your cortex. Killing a few percent of these neurons will probably not make that concept less salient, and will almost certainly not erase it altogether. Of course, when you get older and more neurons die do to natural aging, those missing hundreds of cells might become more important, but even then the loss (hopefully) won't be problematic.

2) Life is meant to be enjoyed. Or in other words, the fun you're having now will be more rewarding to your outlook in twenty years than a tiny bit of extra neural plasticity would be. You'll have great stories to share with your friends, be able to understand why your children are choosing to use drugs, and have memories of how wonderful an enhanced state can be. If you follow your government's advice and avoid drugs entirely, all you're going to get out of it is a tiny bit faster reading speed when you're 60, or the ability to remember a few more people's faces from high school. Relatively great extra enjoyment from drug experiences far outweighs very, very slight extra enjoyment of Real Life.

3) Brain damage leads to physiological diversity. This one takes some explaining, but I think it's conceptually correct. First, being different from one another makes us more interesting as people, and more likely to view reality at a unique angle of our own. Next, all of our diversity in personality and thought is due to our brain and the connections within it. Also, damage done by drugs is more specific than degradation caused by aging -- it's often totally different in location and effect. Finally, not very many people (less than five percent of the population) use drugs that will change their brain's structure. Putting these together, using drugs will change your brain, which will make it different from other peoples', and thus will help your personal outlook be different from the rest of society's. Diversity leads to change, and change leads to advancement of human culture. Hooray!