You grab the IV bag and walk over to the holding pen. The man's eyes go wide. "Wait... wh.. what are you doing?"
You press the IV into his arm. He struggles, but the holding pen is too small. He whimpers as he watches the fluid drain. "Well, you see, I don't know if I can trust you if I just let you out. This way, there's, let's say, some extra incentive to get the antidote right. After all, you, me, and the other 'raptor there will all be using the same antidote. And I will always be watching you. If you make a wrong move, I will tranquilize you and use some of what's in these other IV bags, the results of which might be rather undesirable."
"But that can be deadly mixing stuff like that without knowing what you're doing! And what do you mean, we'll all..." You cut him off.
"You, my friend, will get the first third of the antidote. And as for mixing these being dangerous, it will only concern you if you misbehave." You look at the clock. "Now, your changes should start soon." Minutes later, the man begins to change just like you did. When it finishes, you and he (she) look just like twins. A frightened feminine voice calls out from the cage, "Okay, now will you let me out?"