Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that PJ ingested two 200mg (400mgs in total) Naproxen tablets.
Naporoxen is a nonsteroidal anti-inflammatory that we do not use in dogs due to the high incidence of side effects including gastrointestinal bleeding, ulcers (sometimes perforating) and kidney damage even when given appropriate doses.
The dose listed in Plumb's Veterinary Drug handbook in 2mg per kilogram (2.2 pounds) of body weight every 48 hours.
In a dog his size (100 pounds/ 2.2 pounds per kilogram = 45.45kg) a usual dose would be 91mg, (45.45kg X 2mgs per kilogram = 91mgs), so he ingested roughly 4.4 times a normal dose if he ingested 400mgs.
A likely lethal single (meaning given all at once) toxic dose of Naoproxen in dogs is 35mg/kg, or 1,590mg or 8 tablets of the 200mg size for a 100 pound dog.
Peak blood levels are within 3 hours of ingestion so these levels should have occurred the first night and would have explained his repeated vomiting and loose stools.
Toxic side effects are abdominal pain, vomiting, gastrointestinal ulcers which can perforate, dark tarry stools, weakness, anemia, and kidney damage shown by increased kidney waste product levels (BUN and creatinine) on blood testing.
Unfortunately the half life (the amount of drug metabolized and decreased by half) in dogs is very long, 72 hours, and as such the effects of the drug linger. It is likely he still has high levels of the drug in his system, even now.
While he definitely ate a toxic level, it may not be lethal IF he had normal organ function to start, and he gets good supportive care.
He should see his veterinarian for an exam and treatment based upon how much damage has been done. Your veterinarian will examine him, look for blood in his stools and urine, test his kidney waste blood levels and urine for signs of kidney damage and get him started on sucralfate which is an ulcer coating drug, as well as an acid reducer such as famotidine or omeprazole. They probably will want to put him on intravenous fluid therapy to flush out any remaining drug and support his kidney function. If he is continuing to vomit they can give him injectable anti-nausea drugs. Vomiting is dangerous because it leads to dehydration, which will only further stress his kidneys, and contribute to esophageal ulcers developing.
They will also likely recommend feeding a very bland diet.
At home now you can start either:
1) Pepcid-ac (famotidine) at a dose of one 10mg tablet per 20 to 40 pounds of body weight every 12 hours. (Three of the 10mg tablets for a dog his size).
2) Prilosec (omeprazole) at a dose of one 20mg tablet per 40 to 80 pounds of body weight every 24 hours. (Two of the 20mg tablets for a dog his size).
You should also start feeding a bland diet mix of 1/3 boiled, lean hamburger (or boiled, white, skinless chicken) and 2/3 boiled white rice. Give small meals several times a day. Feed the bland diet for several days, then if his vomiting stops you can slwoly start mixing in his regular diet and slowly convert him back.
But ideally this is something he should see his veterinarian for treatment for to avoid a possible perforating ulcer and/or kidney failure.
Best of luck with PJ, let me know if you have any further questions.