Medicine Garden Blogging: There's an interesting case study in this week's New England Journal of Medicine about a woman who mistook foxglove for dandelion greens (requires subscription), and ate them in her salad. Foxglove is one source of the modern heart medication, digoxin. It's very potent in its natural form, so she ended up in the hospital with cardiac toxicity. Other garden plants and animals produce a similar substances:

In addition to foxglove, several other plant and herbal sources of cardiac glycosides may be detected by serum immunoassays for digoxin or digitoxin (as in this case), including woolly foxglove (Digitalis lanata), ornamental oleander (Nerium oleander), yellow oleander (Thevetia peruviana), squill or sea onion (Urginea maritima), lily of the valley (Convallaria majalis), and ouabain (Strophanthus gratus). Another source of cardiac glycosides is venom extracted from skin glands in certain species of toads (Bufo marinus and Bufo alvarius). This compound has turned up in some aphrodisiacs and Chinese medications (e.g., chan su). Ingestion may cause symptoms and clinical findings similar to those of digitalis overdose, and deaths have been reported.

They may be natural, but they're still poisonous.

ADDENDUM: More garden medicine here.