animals

The pain relieving agent is Resiniferatoxin. From Wikipedia'

Resiniferatoxin (RTX) is a naturally occurring chemical found in resin spurge (Euphorbia resinifera), a cactus-like plant commonly found in Morocco, and in Euphorbia poissonii found in northern Nigeria. It is a potent functional analog of capsaicin, the active ingredient in chili peppers.

At 16 billion Scoville units, resiniferatoxin is rather toxic and can inflict chemical burns in minute quantities. The primary action of resiniferatoxin is to activate sensory neurons responsible for the perception of pain. It is currently the most potent TRPV1 agonist known, with ~500x higher binding affinity for TRPV1 than capsaicin, the active ingredient in hot chili peppers such as those produced by Capsicum annuum. For rats, LD50 through oral ingestion is 148.1 mg/kg. It causes severe burning pain in sub-microgram (less than 1/1,000,000th of a gram) quantities when ingested orally.

That sounds like a hell of a thing to use for pain.