4. You Could Get Addicted
Which isn't in and of itself a problem, since I have a whole inventory full of drugs, plus like every dead enemy has one or two on them, and there are safes and chests everywhere, and if I'm really running low I can just crouch and steal a bunch from a store, but actually, I've got way too many bottle caps that I never spend, so I could actually just buy more, so really there's no issue of me ever running short, but the problem is that if I do get addicted to drugs, my stats will take a hit, and it would really suck to be out in the wasteland and suddenly have my strength take a dive and I'm over-encumbered and need to drop loot, especially since I already spent like twenty minutes paring down my inventory to just the stuff I absolutely need or want to sell, and it would suck to ditch something valuable just 'cause of a stupid thing like a drug addiction.
3. They'll Make Bees Come Out Of You
Bioshock almost, almost made drugs cool. Sure, it's pretty horrifying to jab a big rectangular syringe into your arm every few minutes to pump yourself full of magic stem cells, but if it meant that you could set things on fire with your mind, who wouldn't? The entire game revolved around taking drugs, acquiring materials to synthesize drugs, and figuring out how to make better drugs, and every drug you took made you stronger, badder, and slightly less terrified. Even the morality system didn't even question whether you were going to take cool drugs, only how cool those drugs would be. So you're chugging along, shooting up, lifting objects with your mind and blasting lightning from your fingers (all thanks to drugs!), when you come across a new drug: "Insect Swarm." Having already taken more substances than Jim Morrison on a good weekend, you shove it in a vein without a second thought, only to discover that this particular drug makes bees come out of your arms. And not even when you want them to, either: these bees come and go as they please. Also, your arms become beehives. Also, and I can't stress this enough, bees come out of your arms, because you did drugs.
2. You Might Need Them
No matter how many drugs I have at a given time, no matter how abundant the supply or readily available the product, no matter how helpful they would be here and now, video games have taught me to never, ever do drugs, simply because you might need them later. I could go ahead and pop one of my 100 buffouts, but what if the next supermutant I come across takes all 100 to defeat? Why would I waste a rare candy on a Pokemon in my current roster if I'm pretty sure I'm only minutes away from catching a better one that needs it more? What if I end up quaffing all of my potions during what turns out to be an unwinnable boss fight and then I'm all out of potions, just because I didn't have the good sense to wait? And, knowing all this, why would I use some of my drugs to enjoy this pizza if I might someday need to use all the drugs there are to enjoy every pizza there ever was and will be?
1. Because Winners Don't Use Drugs
And you're a winner.