[Mezzie]

In my case, I use drugs because I have MS and if I didn't, the only signals I ever get from my body are constant pain.

Some occasional drug use reminds me that my body doesn't exist solely to hurt me, that I can still experience pleasure, and that it's still worth trying to find things I might like/be able to enjoy since many of my previous physical pleasures aren't possible or are less enjoyable than they used to be. Without them, my physical existence is only pain, and that's depressing. Being depressed is way worse on my cognitive abilities than doing drugs once in a while.

This is true for most people who use regularly: The drugs are a harm-reduction measure. If weed fucks you up (-10 cognitive ability) but also gets rid of your anxiety (-20 cognitive ability), it's still a net win.