Drugs:
Agent behaviour / performance can be altered by adjusting their drug levels. This side of the game is very useful, but is often misunderstood, or largely ignored by players trying the game out. The manual explains the bars in more detail (see pics), but to explain them with their most obvious behaviour changes:
Adrenaline (Red) —> Faster/slower movement
Perception (Blue) —> Higher/lower accuracy
Intelligence (Brown) —> Higher/lower initiative
High adrenaline: used most of the time, just to get around the map.
Low adrenaline can be worthwhile for increased health regen later (with V3 Chest).
High accuracy useful most of the time.
Low accuracy can be useful for covering fire.
High intelligence useful for guarding areas (agents will open fire on enemies by themselves).
Low intelligence useful for covert missions (agents will not fire unless you make them).
Killer tip: Left + Right mouse buttons = Panic Mode. Maxes all drug levels.
Used in firefights. Essential tactic.
However, the longer drugs are used (bar to right side), the higher the agents tolerance (white line), until the drug has little effect. If that drug is not needed, the tolerance line can be slowly brought back by moving the bar to the left side. Tolerence is controlled better with upgraded heart/brain mods.