In message , Andy Mabbett
writes

I recently bought a Black Elder (Sambucus niger) for friend who's
growing a bird- friendly garden. She also has two dogs, one a puppy.

She was concerned to read on the label that the plant is "harmful if
eaten". Since both elder flowers and elderberries are a food, what's
harmful? Is it the bark or sap, or something else? Or simply
over-cautious labelling.

There was a thread here on the topic recently. Elderberries contain
cyanogenic glucosides, from which cyanide is produced. Google for
sambunigrin.
--
Stewart Robert Hinsley