I had a thought while reading The Wheel of Time a while back. The series is based on a world with cyclic time, with seven history-destroying cataclysms separating the ages from one another. Some ages have technology, some have magic, some have some combination of the two.
I noticed that there seem to be a lot of fairly effective herbal medicines in use. These must be the result of some genetic engineer thousands of years back deciding to splice the genes for every useful pharmaceutical into various plants. While the drug industry wouldn't survive the fall of civilization, the plants would.
That's not a bad idea.