Here is my ideea:In order to make poison you either buy weak-mediocre poison from a shop and to make really strong poison that kills the victim you would have to collect poisonus  plants and mix them. There would be rare, easy to find and suepr rare poison plants and there will be many mixes. to make a cure for such a poison you would have to get the plants and put them togheter with an antidote.In order to know what plants were used you need knowledge of what the effects of that plant are.