Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that Cash was given a small dose of Naproxen.
Naproxen 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.
A dog that weighs 27 pounds (12.27kg) should not take a dose that exceeds 24.5mg, so if you gave 250mgs that is much, much too much medication for a 27 pound dog. It is roughly ten times what should have been given.
A single (meaning given all at once) very toxic, possibly lethal, dose of Aleve in dogs is 35mg/kg, or 429mgs
Peak blood levels are within 3 hours of ingestion so it is much too late to induce vomiting now.
While this may not be a lethal dose for your pup, I would definitely expect side effects given the amount he was given.
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. Toxic levels can stay in the body for several days.
At this point because it is too late to induce vomiting, it is in his best interest to be seen by a veterinarian on an emergency basis and start treatment as a way to try to prevent organ damage and perforating ulcers.
The 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 vomiting they can give him injectable anti-nausea drugs.
They will also likely recommend feeding a very bland diet.
If there is no emergency care available 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.
2) Prilosec (omeprazole) at a dose of one 10mg tablet per 20 to 40 pounds of body weight every 24 hours.
Then push fluids and have him seen by his regular veterinarian immediately to tomorrow morning.
Best of luck with your girl, let me know if you have any further questions.