The more often it is used, the more of the goo is needed to get the same effects yet the recovery and exhaustion effects are greatly increased.
Additionally, after the initial exhaustion wears off, the goo could have an addictive nature that results in mored days of sweats and extreme weakness that lasts longer than the initial recovery. That would make the goo a dangerous thing to use unless they have enough quantities to keep those that use it going and are relatively sure of the safety for the two recovery periods.
NPCs (Minor) (Mystical)