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I would like to help you and Pinky with this question, but need a bit more information in order to better assist you.
You can buy it at your local pharmacy.
Legally, I cannot prescribe medications for a dog that I have not seen!
However, I can tell you that in a 12 lb dog that I had examined, I would use ½ of a 10mg tablet every 8 hours for 4 days. Then, I would drop to ½ of a tablet every 12 hours for 3 days. Here is more about famotidine, including dose:
Dogs eat the strangest things - plastic bags, children's toys, bones, bits of towel, socks, rocks and other things. Often, these foreign bodies pass through the intestinal tract, but sometimes they do not. They may get caught in the stomach or the small intestines. The symptoms of a GI foreign body are generally vomiting, loss of appetite, depression and dehydration. If the bone is caught in the stomach or small intestines, it might explain the symptoms that you are seeing.
In the case of an obstruction, surgery is often needed to remove the foreign object. I will include further information about GI foreign bodies: http://www.michvet.com/library/surgery_gi_foreign_body.asp http://www.petplace.com/dogs/gastrointestinal-foreign-body-in-dogs/page1.aspx
If I examined your girl and was concerned about a foreign body, I would probably recommend x-rays to see if a foreign object were visible. A bone would show up very well on x-rays.
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The above is given for information only. Although I am a licensed veterinarian, I cannot legally prescribe medicines or diagnose your pet's condition without performing a physical exam. If you have concerns about your pet I would strongly advise contacting your regular veterinarian.
Then, assuming no more vomiting, there are some things you can do at home until you can get her in to the vet:
1. WITH-HOLD FOOD until it has been 24 hours since she last vomited. This gives the intestines a chance to rest and heal.
2. When she is fasting, she can have lots of clear fluids. Wait until it has been 4 hours since she last vomited, and give small amounts of plain water to start.
So, water is fine, but if she keeps that down for 2 hours, she can then have pedialyte, Gatorade, apple juice diluted 50:50 with water, or chicken or beef broth diluted 50:50 with water. Give the fluids in small amounts frequently. For a dog this size that means about ¼ to ½ cup an hour.
3. After 24 hours without vomiting, you can start your dog back on a bland diet.
For patients that I see, I recommend a mixture of 75% cooked white rice, and 25% low fat protein. For the protein you could use extra lean ground beef, boiled with the fat scooped off, or chicken breast boiled with fat scooped off or even scrambled egg cooked without fat in the microwave. Feed small frequent meals. For a dog this size, I would suggest 2 or 3 tablespoons every 3 to 4 hours.
4. After 1-2 days on the rice mix, you would gradually change your dog back to the normal dog food.So, on day 3, give the rice mixture, but bigger meals, spaced further apart. On day 4, mix a little tiny bit of the normal food in there, and decrease the frequency so it is down to 3 meals or so. And so on.
5. Keep your dog as quiet as possible - just out to relieve herself and back in.
If your dog continues to vomit, develops blood in the stool, is lethargic or shows signs of abdominal pain, please contact a veterinarian promptly.
I do hope that this helps you to help your dog!