I am sorry to hear that Newt ingested a 500mg dose of Keppra (Levetiracetam) which was meant for another of your dogs.
This drug is used as an anti-convulsant medication. It is a fairly safe drug and is often used because it isn't metabolized by the liver, so older dogs seem to tolerate it well.
It should be dosed carefully in dogs that are taking nonsteroidal anti-inflammatories, like Rimadyl.
The usual dose is 5 to 10 mg per pound of body weight (10 to 20 mg/kg) three times daily (every 8 hours).
So 500mgs would be appropriate for a dog 50 to 100 pounds of body weight.
One of the most common side effects of Levetiracetam in dogs is sedation, but we can see nausea and lack of appetite as well.
Since your fellow weighs 16 pounds he took roughly 3 times a usual high dose. Since he is healthy otherwise, not taking any other medications, I would expect that Newt may be quite sedated, but there shouldn't be anything too serious.
You may not truly need to do anything other than keep him away from stairs in case he gets very sleepy and is a bit uncoordinated.
If he is on other medications or has chronic health issues then please let me know and inducing vomiting may be better.
Or if this just happened (less than 1-2 hours ago) and you have fresh (unexpired) 3% hydrogen peroxide you can choose to avoid trouble by inducing vomiting.
Induce vomiting by giving him 3% hydrogen peroxide at a dose of 1 tablespoon orally, or mixed with a little ice cream or bread chunks, then take him out and run him around to get the peroxide fizzing.
If no vomiting in 10 minutes repeat the peroxide dose once.
If he vomits and clears his stomach (vomits until only mucous is brought up) then to relieve the irritation from the peroxide you can give either:
1) Pepcid ac (famotidine) at a dose of one half of a 10mg tablet per 10 to 20 pounds of body weight every 12 hours
2) Prilosec (omeprazole) at a dose of one half of a 10mg tablet per 10 to 20 pounds of body weight every 24 hours
These are acid reducers and should help him feel less nauseous.
Please let me know if you have any further questions.