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It is used in dogs to calm them down.
You can read about its use in dogs here
which states 250-300mg in large dogs is a typical dose.
So if that is the dose range you should be all set.
It also says "Animal studies involving very large doses for 26 weeks have not shown any serious effects."
If you are really still worried you could induce vomiting if she just got it within the last hour.
You can get the dog to vomit it up by using fresh 3% hydrogen peroxide which you can read about scrolling down on this pagehttp://www.petplace.com/dogs/how-to-induce-vomiting-emesis-in-dogs/page1.aspx Do this only on an easily cleaned surface and some people have the dog stand in the tub to vomit so clean up is easy.Hope this helps you!