Leslie ~

Hope Vet #2 gets to the bottom of poor Minion's tummy upset.

Here's some info I found on the Bayer Animal Health Division web site...
Bayer/Advantage web site - Label page

Active Ingredient in ADVANTAGE (Mfg by Bayer AG - the Asprin people)

Imidacloprid; 1-[(6-Chloro-3-pyridinyl) methyl]-N-nitro-2-imidazolidinimine = 9.1% be weight

Inert Ingredients = 90.9% by weight

Total = 100%
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They also have a Phone Number
1-877-258-2280
that will get you or your Vet to a Vet Tech who can offer suggestions.

I called and got the following:

Advantage is NOT absorbed into the cat's skin or blood - it works by being carried over the cat's skin on the cat's normal skin OILS - all externally. The Imidacloprid then effects the nerves of the flea - but does not effect the cat or humans.

Other than cases of skin irratation - they claim no know adverse reactions. Test animals showed no reactions to ingesting the ingredients.

The material can be washed off the cat by using warm water and a Dish Detergent - which will remove the skin oils from the cat.

The Tech IS available to consult with your Vet.
The Tech suggested that Minion may very well have an illness or internal blockage not related to the Advantage - and suggested further testing, x-rays, or ultrasound should be considered.

I asked what might happen if Minion had licked Advantage off one of the Dogs...
Not Probable was the answer - the stuff tastes Really BAD to cats & dogs - and the normal reaction to licking it is a LOT of spitting and/or foaming at the mouth. Any resulting stomach upset would not last for several days.

After doing Google searches - I haven't found references to other pets suffering upsets or allergic reactions to Advantage.

Sorry this hasn't been of much help.
Prayers offered that the Vet can resolve poor Minion's problem today!

Hang in there, Minion!
We're all praying for ypu!