Ok so I had an idea for a mechanic that I like: each time the PC's take the drug they get a certain boost to all stats and they get powers. However, once the drug wears off they notice not only does the boost wear off but it takes some of their original stats with it. Once they take the drug again they are back up to their boosted state but once it wears off they find they are even further depleted. I'm thinking of it almost like if every time Billy Batson says the magic "Shazam" he's buff and strong but once he turns back into regular old Billy he's gaunt and has hair falling out. So, it's a multiform with a funky custom limiter, that much is obvious.
That's all well and good but as I thought about it more something bothered me: if they can just pop the drugs to regain their superheroic stats it doesn't really matter how bad their stats are in their civilian identity. They'll need to pop the drugs if they want access to their power, so they won't really have to worry about their decay because they won't be caught out without their powers... unless they burn through the drugs in their system mid-combat.
This is what I'm trying to figure out: how long should the powers last? A round? a few phases? What I want to achieve is that their multiform only lasts a little while in combat. This forces them to resort to risky methods to end fights early OR to snort the possibly contaminated drugs off the floor, quaff some unstable reagent, or spend their move action frantically searching for the villain's stash in order to continue fighting. If they don't get their hands on more then they risk reverting back to their increasingly frail civilian identities.