I cannot find the rules for focus addiction. I know they exist, I'm pretty sure I've seen the heading, but right this second I am having a brain fart. Help me out, please!

Street Magic pp. 26-27 is a place to start. That's where you find the negative quality.

Digital Grimoire they come up with some really basic guidelines. I think it is actively running X2 magic force worth of focuses at the same time, leads to focus addiction.

I personally suggest using this as a "rule", not a "guideline", with possible additions based on duration of foci active (hours in excess of Magic, perhaps). Regardless, it should be clear ahead of time exactly what can/will cause Focus Addiction.

How do you roleplay the craving if you always have your foci available for you?

There are a wide range of reasons on why you'd turn them off, you are in a high security area, going through a ward, you don't want to stand out etc. An addict will likely use them in those situations as well. Also just being addicted to focuses used to wreck your magic rating, I am not sure if that made it to 4e.

You just sit there, stroking your foci in a really disturbing manner. Sometimes you call it "my preciousssss".

K.

and I'd mostly throw the rule out, as no one else has to make addiction tests to use tools/skills they've spent karma on.

Same here, actually. I file focus addiction in the same bin as "everywhere has background count" - ways to sit on magician players when you don't know how to stop them.

K.

It's another nagging feature, yeah. Sure, you can come up with justifications, but it's just not cool.

You should punish characters for using "forbidden" powers, such as borrowing power from icky spirit pacts, or overuse of possession perhaps, and using drugs to augment your abilities; not for using the "straight and narrow".

The only addiction I dislike more is the Matrix Addiction. I mean, this is the 2070s, that kind of behavior is well-adapted to a society that spends most of its time in AR anyway.

Just because the 'substance' being abused is everywhere and has been for all the character's life doesn't mean it's not addictive. If anything that should make it even more of an issue when the sprawler ventures into dead-zones for extended periods. Also, just because society has evolved a certain way doesn't make its conventions healthy – this is a distopia after all.

You could always argue that all characters (with some rural-born exceptions) have this dependency and forget the quality altogether.
In fact now that I mention it, I quite like the idea ...

That's the whole point; the matrix is so pervasive that this is hardly an exceptional quality that deserves to be noted on a character sheet. It also smells faintly of parents telling you to go to bed and not to play computer games so much.

By the rules, addictions rarely have anything at all to do with the substance so much as the craving. There's plenty of drugs that have no noticeable side-effects (G3) and some that even include an automatic addiction quality despite that fact (Betel). The only time these (and, really, any addiction) come up during a run is when the runners are somehow isolated and cut-off from their normal routine. I mean, even if you have an addiction to something that leaves you all but crippled after it wears off (Red Mescaline or Novacoke), it's easy to take care of your normal cravings during your downtime for mild and moderate addictions and thus render those secondary drawbacks null and void.

The problem isn't what the addiction is to, but how you isolate the character from it. It takes some seriously extreme circumstances to do that, especially since most players won't take addictions to stuff that'll likely get you killed if you take it on even a remotely regular basis (like Kamikaze) anyway. Short of getting arrested, placed in orbit, or getting kidnapped, there's not much you can do unless they were foolish enough to not have any contacts who could supply them, and don't own or even carry a stockpile of the drug on them.

That said, addictions to foci, the Matrix, and even goofy things like sex or alcohol shouldn't be any more of a lame way out than some of the more hardcore drugs. The craving is the drawback, not the substance. Figure out a way to fix that and those other addictions would work out just fine as a real negative quality; not that they're really any less worse than an addiction to, say, Psyche or Guts.

That reminds me, my technomancer hasn't chewed betel in a while. Thanks for the memo.

To be fair, there are two matrix related ones. Media Junkie (which I'm sure most of us suffer in some way, shape, or form) and addiction to Hot-Sim (TMs are immune to this). From the description of what hot-simming is like, I completely understand it being capable of addiction, the speed, the sensation, the freedom. Psychological dependence is still an addiction.

A species as complex and long-lived as humankind needs thousands, or tens of thousands of years to adapt physically to a changed environment. We still crave salt and sugar far more than we need or is healthy, even though they both have been available in virtually unlimited quantity for over a century.