Also known as Hogweed and closely related to the inedible Giant Hogweed. Care must be taken when handling this plant. Under certain conditions, the plants produce high levels of furanocoumarins which, is carried in the sap and, if splashed onto skin, cause photosensitivity. This leads to blisters and burns under UV exposure which can be painful.
I have read reports of another Heracleum species, H. sphondylium, causing celery-like allergic reactions in people. Since celery is one of the leading causes of allergy, caution is advised when consuming this plant.
That said, it must be noted that the leaves are very delicious. They are almost succulent in texture and they saute very well. With a little butter, they can be slowly sauteed to a wafer thin crisp - it takes a while but it's worth the wait.
Cow Parsnip leaf tempura is excellent. In fact, any tender part of cow parsnip fried in a tempura batter and served with wild garlic aioli, or simply simmered and buttered makes a humble and warming side dish.