Please get to the vet ASAP, I found the following about Comet and dogs:
The concern is that this product is irritating to the stomach and mucus membranes in the mouth and throat. It can cause ulcers in these tissues.
The pain, inflammation, redness and swelling may not be visible for 2-4 hours after the Comet was eaten. The full extent of the injury won't be known for 12 hours after exposure as the corrosive burns take that long to show up.
Treatment of burns in the mucosa should include antibiotics, pain medication as needed, gastrointestinal protectants (e.g. sucralfate), anti-inflammatories (corticosteroid use is controversial) and general supportive care.
In cases with severe oral burns or esophageal burns, your veterinarian may have to place a feeding tube into the dog's stomach so that food and water can be given to him through this tube while the mucosa heals. This can take weeks. There is a risk of a stricture or narrowing forming in the esophagus.
Treatment for a stomach ulcer or burns of the mucosa in the mouth and throat involves giving 3 drugs:
1. Sucralfate (Sulcrate, carafate) which acts as a bandaid to stick to the areas that are ulcerated. Thus, it is very important that this medication be given on an EMPTY stomach.
Here is more:
http://www.petplace.com/drug-library...ate/page1.aspx
2. Antacids such as cimetidine (Tagamet), famotidine (Pepcid) or ranitidine (Zantac).
Here is more about Tagamet:
http://www.petplace.com/drug-library...met/page1.aspx
More about Pepcid:
http://www.petplace.com/drug-library...cid/page1.aspx
And more about Zantac:
http://www.petplace.com/drug-library...tac/page1.aspx
3. Antibiotics to prevent bacteria entering the blood stream through the ulcer in the stomach.