Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that your pup ingested two Advil tablets.
Regular Advil contains 200mgs of ibuprofen per pill. At 18 pounds he is about 8.2kgs, so if he ingested two tablets he ingested 400mgs or about 48.8 mgs per kilogram of body weight (400mg/8.2kg = 48.8mg/kg) which is a toxic amount.
From the ASPCA database:
◾25-125mg per kg of body weight - symptoms include vomiting, diarrhea, abdominal pain, loss of appetite, stomach ulcers;
◾more than 175mg per kg of body weight - the above symptoms plus blood in the stools, acute kidney failure;
◾more than 400mg per kg of body weight - the above symptoms plus seizures, coma, disorientation;
◾more than 600mg per kg of body weight - death.
Given the amount he ate I think you could see vomiting, diarrhea, abdominal pain, loss of appetite, and stomach ulcers.
Ideally he should see his veterinarian and be prescribed something to combat nausea (Cerenia), fluids, and something to coat his stomach (such as sucralfate) to guard against ulcers forming.
At home now to try and decrease acid production in his stomach and lessen any possible symptoms you can give either:
1) Pepcid ac (famotidine) at 1/2 of a 10mg tablet per 10 to 20 pounds of body weight every 12 hours
2) Prilosec (omeprazole) at 1/4 of a 20mg tablet per 10 to 20 pounds of body weight every 24 hours
These are acid reducers that are found over the counter at the drug store. They should be given for two to three weeks.
To further decrease the symptoms you can feed him a bland diet for the next several days. A homemade bland diet is a mix of 1/3 boiled minced white skinless chicken or lean boiled hamburger and 2/3 boiled white rice mixed with some low salt chicken broth to make it easy to lap up and swallow and get in plenty of fluids. Feed small meals frequently.
Things to watch for that mean trouble include continued vomiting, especially blood tinged, dark, black or tarry stools, or frank blood in the stools, lack of an appetite, increased water consumption and urination.
Please let me know if you have any further questions.