Combat Stimulant Tips
Enter inventory screen and right click, then use. It costs 2AP in combat, but will give +4 AP so you get a net +2 AP on the turn that you use it, just make sure you have that 2AP saved up to use it to begin with or do it at the beginning of the turn.
Then I think you get +4 AP next turn since you don’t need to spend 2 AP to use it again (I’m not actually sure on the 2nd turn amount, maybe someone else can confirm, I assume it’s the same as 1st turn though).
Then on the 3rd turn your merc will become tired. So it’s a useful thing in do or die situations but the cost of having to rest the merc and waste time after the fight is kinda hefty.
One of the most useful situations I’ve found for it is when I wanted Barry to use a couple of AP to move into a position to bombard someone, then still have enough AP to throw 2 explosives on that turn then use his post-explosives free turn to move back to cover (Corazon’s bunker where enemies are entrenched behind the walls of the room with single entrance).