First, I googled around on this subject and found a lot of information on vitamins for someone withdrawing/detoxing to take. Not muich on what a CURRENT user should take.
But SWIM smokes black tar heroin and doesn't plan on quitting right now. He just got a job, and has an income for the first time in months. (even before he used heroin he was out of work). He's on the verge of moving into his own place with his lovely girlfriend, things are going good. He doesn't wanna fuck up his employment by having to miss days or get all pissy withdrawing.
So while he's using heroin he wants to keep his body as healthy as possible. He's slowly been exercising more and more, he drinks lots of water and very little soda/caffeine (he drinks decaf coffee). What other steps could he take to help keep his body healthy while using heroin? What vitamins should he take every day, and why? (he likes to know why to pass on this advice to other heroin users, and so he has an explanation so he doesn't sound like he's talking out of his ass).
He's heard vitamin C is definitely a good one, but what else helps? Stuff that helps muscles, stuff that helps the immune system, stuff that helps keep his brain function normal, etc?
Any help would be appreciated

Thanks in advance, you guys have been great every time I've needed you