Alcohol - if Harkness was a human, he’d be a nonfunctioning alcoholic given how much he drinks. He’s got a great metabolism, but he’s not happy about it. He only started drinking hard after the LW reawakened his memories. There was a reason he wanted a mind wipe (thanks for nothing Pinkerton) and with all those memories back he chases the ever elusive ‘drinking to forget.’ He literally can’t, but it doesn’t stop him from relentlessly trying. He’s actually been called out for drinking on the job back in the City, but he tends to deal with that aggressively (of course) and challenge whoever calls him on it to do a better job at security. They usually back down, but again, he’s a big dude.
Smoking - again, if he were human the amount that he smokes would kill him slowly and painfully. He’s very rarely seen without a cigarette dandling from his lips. This habit has a less nefarious origin, in that when he was A3-21 he noticed that a lot of humans smoked and it was a way to blend in. When his memories were buried, that instinct remained. As a synth, he doesn’t do ‘blending in’ (or anything) by halves, so naturally he smokes constantly. I pity whoever has to stand next to him bc you’re gonna get secondhand smoke out the wazoo.
Caffeine - same as the smoking thing. People are always talking about how they “need their coffee” so he mimicked that by being grouchy until he has coffee. It doesn’t do much of anything for him, but he drinks it anyway. Please no one introduce him to Irish Coffee, or we’ll really have a problem on our hands.