Nah, he should be fine. I’m 99% certain that bcp are on our big list of nontoxic ingestions. Eating an entire pack could upset his tummy, though. I’d go fairly light on the breakfast for him today.
Under the light of the next full moon, your dog will change into Ally McBeal. The rampage will not end until dawn, when your good old lovable mutt will return.
The only possible toxicity would be if the placebo pills are iron supplements and then it would depend on if the amount of iron is over what would be a normal dose for the dog, which might only be a problem for a very small dog. They might also have some nausea or diarrhea from it. Unspayed female dogs could have a little uterine bleeding/spotting but it’s not usually serious. Birth control pills are generally regarded as safe.
It always gets me how people leave these things lying around in easy reach of their pets and small children.