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Hi there. Thanks for requesting me.
Although Metformin (also known as Glucophage) doesn't usually work very well to lower the blood sugar in dogs, this is a very small dog with a very large dose.
Normally I'd tell you to induce vomiting at home with hydrogen perxide, but since it's been over 4 hours since she ingested it, there's no use. The tablet is already dissolved and since she's having some shaking, it's clear that her body is processing it.
Something you can start with is to rub some maple or Karo syrup on her gums. Even honey can work. We want to raise her blood sugar to counter the Metformin.
If that doesn't help, then you absolutely need to have her seen by an emergency vet in your area so that they can monitor her and help get her blood sugar levels raised to a safe level. If you're not sure where an emergency vet is in your area, you can go to: http://www.vetlocator.com and plug in your zip code. They'll point you in the direction fo the one nearest to you.
I hope this helps.