I have this under the "myths" - with a justification:
We nurses used to say this so that you patients would leave us your chocolate
gifts.
Always gets a laugh - and as it usually has been at the end of a series of "can
you eat...." series of questions, it's a good way to get people to RELAX.
Of course there are some babies that are sensitive to this as to any food
product - especially "milk" chocolate - or the caffeine it contains.  Luckily
for me, my kids did fine even though I love M & M's.
Jeanette