Nice work guys, an addition like this could pave the way for a wave of effect based items mimicking real life drugs. Caffiene is a big one. I would say another would be tobacco considering it helps workers the world over stave off hunger by suppressing appetites. A few others that could be introduced could give combat related effects:
Amphetimines
Opiates
PCP
Amphetamines could increase movement speed, mining speed, reduce hunger and maybe have an effect on damage. Opiates could reduce the amount of damage a player takes. PCP - well even though its supposed to be a horse tranquilizer I once saw a guy on PCP punch through a thick glass display window, breaking every bone in his hand but still going. PCP would probably give the greatest effect at the cost of completely losing your mind (said guy who punched through the window went on to hold his girlfriend hostage and eventually was shot by the police).
Also these drugs were used by militaries extensively in WWII (Japanese AA gunners given meth to stay awake, all soldiers given morphine). They are even used in the current war on turror in the middle east. I remember reading about how in the battle for Fallujah soldiers would find dead insurgents with heroin needles, all kinds of pills, PCP, etc. They would do all the drugs available to them and fearlessly rush into combat whilst ignoring pain that would send a sober person into shock. Completely outnumbered, surrounded and outgunned they managed to hold out for a while and even inflict some serious casualties on the attacking Marines.
That being said there ought to be some serious drawbacks to using the drugs. Maybe a poison effect that takes hold when the drug wears off to slowly damage the player. I'm also wondering if it would be possible to stack effects and if so, make stacking effects have a pretty solid chance of just killing the player outright (especially when mixing stimulants and depressants). An addiction effect would be nice too, slowly damaging players who are not under the effect of a drug that they had taken too many times. Yes, you could just die to escape the effects of withdrawal/addicition but also consider that when you have traveled halfway across a map to participate in a conflict and are carrying all kinds of important gear mitigating the negative effects would be easier then respawning.