Concerning Niko: You can go drinking with friends, but it isn’t shown. You just park outside of the bar, it fades to black and you show up outside again, drunk. The screen whirls and blurs heavily while you stagger and sway, barely able to walk. But if you try to get in the car and drive drunk, Niko will say to himself, “If you drink and drive you’re a bloody idiot!!” If you ignore Niko’s warning you can drive drunk, but the police will be after you immediately, and will most probably catch you. If you don’t want to drive drunk (DUH!), you can wait it out or take a taxi home. There is no way to buy or use drugs in this game. Also worth noting is that Niko does not use drugs or drink in any of the cutscenes, and if asked he will turn people down every time.

But, most every other character is seen doing lines of coke, smoking weed, heroin, crystal meth, and drinking frequently throughout the entire story. Almost all of your missions involve you having to either kill drug dealers, work for drug dealers or steal drugs for your clients. There is much more detail in the "Missions" category, so read a couple to get a better idea on what Niko will be doing concerning drugs.