A sample of the original poison is needed to make an antidote, yes. Otherwise it'd be difficult to measure the effectiveness and what to counteract.

[He clicks the door shut behind them, turning the lock. Yeah, sure, they're totally friends now... anyway.]

Please, there's no need to be so nervous. Have a seat. [smiling, gesturing to the bed] Now, why don't we start with where this venom came from? Is it something from the forest? A creature, a plant?

[pay no mind to the hovering syringe-drone circling around Mizuki, quietly beeping... this is just a very normal medical scan, nothing to be concerned about!!]