[Agema]

As stated above, curare, used by South American natives, was effective on arrows as it killed if it got into the bloodstream. It was harmless when ingested, as it couldn't be absorbed and is (I think) ultimately broken down by the digestive system. On the other hand, you'd need to worry if you tried that and you had a stomach ulcer.

They could also feasibly use a poison they have a natural resistance or acquired tolerance for: lots of animals use poison to kill their prey but don't die when they eat the victim.