My first idea is that they'd have a relatively low damage, but the tranquilizer itself would act as a poison. Roll above DCX or you feel groggy, roll again for the next few rounds to avoid actually taking asleep, successive or later doses increase the DC.

The other relatively elegant option is to have them deal non-lethal damage. Not the best emulsion, but it's a way the system provides to go from 'awake' to 'unconscious', and theoretically to 'dear' if you do with of it.