Bigfish has got it spot on! the sheath covering the barb carries the toxins,its not a hypodermic action as with a bee of wasp.Not too sure how long it is between shedding the barb,i would guess at twice per year.I just remove them as soon as i see them in the substrate,they can be very dangerous to the rays if you leave them in,not because of any toxin that might be left,rays are immune to the venom, but because its easy for a discarded barb to finish up embeded in a rays abdomen,i lost a stunning henlei this way,always check the sand or gravel regularly and dont have it too deep,probably another reason why so many people prefer a bare bottom tank.