Hi,
I didn't really know, either. I found this and if you'll read all of the contents, it seems as though most frogs excrete a substance that is foul tasting and which causes dogs to salivate and foam at the mouth and in future leave the frogs alone. I do know that some are poisonous, but I don't believe we have that kind around here. You might investigate further.
http://www.thriftyfun.com/tf76036554.tip.html