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Luckily, birth control pills are on the non toxic list of things that dogs frequently eat. Of course, you're still going to want to keep an eye open for things like vomiting, diarrhea or lethargy, just to be on the safe side, but I doubt you'll see any of these things.
For future reference, here is the list of non toxic items commonly eaten by dogs. http://www.petplace.com/dogs/nontoxic-items-commonly-eaten-by-dogs/page1.aspx