A well-prepared apocalypse survivor should be thoroughly educated and experienced in herbalism. The healing properties of plants are not only myriad and potentially strong, but they also may be the only form of medicine available once every pharmacy and medicine cabinet has been looted (by violent gangs of thugs who stockpile all drugs in a region and trade them at ruthlessly high rates to those who are unfortunate enough to fall ill after the bombs have fallen).
Ingesting some herbs can be just as dangerous as thugs, however, so the Survivor should be careful. Plants can have multiple, sometimes conflicting, properties when used or prepared in different ways.
For example, Yarrow (Achillea millefolium) purportedly has dozens of effects on people: It has historically been used in a poultice to staunch the flow of blood from wounds (the scientific name Achillea is derived from mythical Achilles, who supposedly carried the plant with him for his soldiers’ wounds (1)), but it can also stimulate circulation and encourage bleeding if ingested. Those are opposite effects, so the Survivor should make sure not to have anything backwards!
Another example: Comfrey (Symphytum officinale), although also very useful as a poultice for flesh wounds and as replacement for toilet paper, has been known to cause kidney or liver failure when ingested (2).
The Survivor should therefore be sure to have positively identified a plant and try it first in small, diluted quantities. Reading as much about these plants as possible before the apocalypse, especially sources by true experts who have personally foraged more than I, would be extremely beneficial.