Personally, I tend to work addictions of any type (as well as fears) with a rule that means that if the character is suffering from withdrawal (or fear), they take a Wp test (Nv in the case of fear) at the start of each turn, and if they fail, they're at -20% to all stats except S & T for that turn due to the addiction/fear affecting them.
So, if the character is withdrawn, then I'd use a rule like that. How to decide if they're withdrawn is another matter. Personally I'd do it off their action rolls (which saves on extra dice rolls). All the dice coming up as 1, two/three "Risky Action" turns in a row, or something. You don't want the effect to be too common.
Obviously, to end the withdrawal, then he has to take the drugs (after he's taken them, I wouldn't then test for withdrawal in the same game.)
Injection... I really have little idea. TheNephew's rules sound like they'd be up to scratch though.
For the effects, I might simply use (part of) the rules for the "Analgesia Infuser" from the Istvaanian sourcebook. +1 bonus to recovery rolls for the rest of the game, but -10 Initiative because of reduced reaction time.
Alternatively, when under the effects of the drugs, the "Pain is Nothing" rule I give to one of my characters may be appropriate - their consciousness value is 3/4 (rather than 1/2) their toughness, and a system shock result simply stuns them for D3 turns.
This is however the character from my collection who has thus far proven to be impossible to (permanently) kill - being a very powerful regenerator. Having "died" three times and been subjected to just about every weapon known to the Imperium as well, pain doesn't mean a whole lot to him any more.
I'd definitely layer "Pain is Nothing" with some kind of penalty though, probably -20% to Initiative. It is not a rule to be taken lightly.