Packing peanuts come in two types: biodegradable, made with cornstarch and other substances -- and immortal, such as those made of Styrofoam. One can be dangerous, the other is merely annoying. How do you know which kind your dog has scarfed down? Drop one in water. If it softens and dissolves, it's biodegradable. If the dog ate every last peanut and you can't be sure, assume the worst.
Potentially No Big Deal
If the packing peanuts your dog ate are the biodegradable kind, it's not likely a crash emergency. These are described by the industry as "safe for your family, your pets and the environment." The most you could expect would probably be an upset stomach. To err on the side of caution, however, call your veterinarian as soon as possible. In the meantime, watch for belly pain and lack of appetite -- which are signs of intestinal blockage -- and diarrhea.
If the packing peanuts your dog ate prove to be the non-dissolving plastic kind, call your vet or the ASPCA animal poison control center immediately. Do not jump the gun and make your dog vomit with any household substance. Your dog may get regurgitate the peanuts on his own; if he does not, you'll want your vet's approval before you induce vomiting.
If more than two hours have passed since the dog had his little snack, vomiting will likely do no good, as the peanuts will have passed out of his stomach into his intestines, and there are only two ways out of there. One is surgery. The other is … well, you know.
Making a Dog Barf
If, and only if, you are instructed to induce vomiting in your dog, follow the vet's instructions precisely, including accurate measurements. Keep some 3 percent hydrogen peroxide or ipecac syrup in your medicine chest and tell the emergency folks which one you have so they can coach you on how to use it correctly. After inducing vomiting, get the dog to the vet as soon as possible for ongoing treatment.
The best way to keep a dog from eating packing peanuts is to keep the two strictly separated. Put the dog outside or in another room when you open packages. Use the water test to identify biodegradable peanuts, and stash them for other uses. If you plan to reuse plastic peanuts, store them in a trash bag or box and put it where the dog can't get at them. For some dogs, this means placing them on a high shelf or in the garage; for others it could mean keeping them under lock and key or off the premises altogether. Whatever it takes, keep packing peanuts from your dog.