I use marijuana. I also use tea. And tomato juice. I'd say it does affect your coding, to the worse. However, it makes your creative thinking and problem-solving more enjoyable, which is a good thing, and positively gives you a boost.

So when I actually sit down to code something I designed prior to this, I won't use it, it makes me slow and narrows the attention scope, which makes me paranoid, which makes me go through my code a couple more times, and definitely more than needed to catch all the trivial bugs.