That glare would probably have more of an effect if Derek wasn't shirtless and in close proximity.
"...I know," Stiles says, obviously distracted, as his eyes move over Derek's skin. "It poisoned you. And it healed you, when it should have killed you. So. Werewolf magic. believe me, I'd much rather test the wolfsbane potion on Peter, but you seem to be ah...evolving faster."