According to the ASPCA they're non-poisonous to dogs / cats / horses but they don't say anything about humans.

I'd think that eating too many of them at one setting might result in lose stools though.
ETA: and since the tile of this thread is "Blueberries" not "Mulberries," I'll let you know that they're not poisonous either ... just like humans, they derive quite a few benefits from Blueberries.
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“Properly trained, a man can be dog’s best friend.”
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