Due to their lack of chlorophyll Anchlies are parasitic plants. They can be vicious little things but they're not evil, it's simply in their nature. Without a host, Anchlies can't survive on their own because they lack the ability to photosinthetize their own food.
What the pills do is provide the Anchlies with the chlorophyll they lack so that they can manufacture their own food and nourish themselves just like any other plant and thus eliminating the need for a host. Giving an Anchlie pills is optional.