Friend, vomiting can be caused by either primary problems in the gastrointestinal tract or secondary to a systemic disease. It will be difficult for me to determine the exact underlying cause without a hands on examination and some testing. But, common causes are: A simple gastritis, dietary indiscretion, ingestion of a foreign body, infections, pancreatitis, etc. to name a few causes. The fact that Skittles is 16 years old we also have to rule out systemic diseases like: Kidney or liver disease, hormonal disorders, tumors, etc. to name a few causes.
In this case, I would strongly recommend a hands on examination with family vet, blood work, urinalysis and +/-xrays as initial screening test to determine the exact underlying cause. In the meantime in any case of vomiting the first thing to do is give the stomach time to rest. This is what you can do:
1- Do not give things like Pepto Bismol or imodium. Start him on Pepcid AC (famotidine) 10mg 1/4 tablet every 12 hours for the next 2-3 days. You can buy Pepcid AC over the counter in any pharmacy.
2- No food or water for 4-6 hours. Need to give his gastrointestinal tract time to rest.
3- After 4-6 hours offer a small amount of water. Wait half hour, if no vomiting, offer small amount of rice (50%) and boiled boneless chicken or ground beef (50%) 4-6 times a day.
4- Follow up with your family veterinarian for a hands on examination. If cost is a concern look into your local ASPCA or Shelter where they often offer veterinary services at a lower cost.
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