Maybe an implant of poison coated in a shell that degrades predictably with appropriate kinetics, and you just swap the implant every few months. Seems like qui

Maybe an implant of poison coated in a shell that degrades predictably with appropriate kinetics, and you just swap the implant every few months. Seems like quite the materials engineering challenge though - it would have to stay intact for a long time then degrade rapidly. In addition to a myriad of other technical challenges (protection from impact, a minimally unpleasant poison, secrecy, etc)