This isn't really true. Trying to heal cancer, AIDS, or something similar is impossible with just herbs and/or magic. And there are many plants that are poisonous and used specifically in curses and nasty spells.
Not everything can be cured with plants, this is 100% true, like aids, smallpox, cancer, etc cant be cured with just a plant.
But many plants can "cure" and effect the ailments of the body. The plants that can do this and effect the body are also all in fact poisonous. Like the Weeping Willow tree which contains a form of neurotoxin in the bark that directly effects the part of the brain associated with headaches (asprin), but when taken in large amounts will eventually numb the brain and cause somebody to go into a temporary state of paralysis. Nearly every medication that is found on the market today is made from poisonous plants, animals, and minerals like lead and mercury (yes these minerals are found in some medications but in trace amounts). BUT these poisons are only used in minute amounts and never large doses for safety reasons.