I have worked out an answer for you ...
1. The best thing to do here is to just sit it out, monitor your dog and see what happens. In my experience, most dogs will get away with this type of thing and you will see no ill effects. This was just a small piece of foil and hopefully your dog bit the foil into further fragments, the chances are it will and it just pass straight through your dog's digestive system.
2. One thing that might help matters is to feed your dog quite a few slices of bread or some mashed potato as it may form round the aluminium foil fragments and cushion them to some extent and so help protect your dog's intestinal tract lining as the foil passes along. This can be very effective.
3. However be aware that though that there is a very small chance that a problem could occur here due to damage to the intestinal tract. So monitor your dog carefully over the next few days for any odd symptoms such as vomiting, lethargy, anorexia, abdominal pain or straining and if such occur contact your vet without delay.
That said, in my experience most dogs will get away with this sort of incident involving aluminium foil, it is a common event ...
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