Friend, a single overdose of fish oil capsules will not be toxic. But, it can cause gastrointestinal symptoms like vomiting, diarrhea, pancreatitis, etc.
I would keep an eye on them if they develop diarrhea or vomiting this is what you can do:
1- Do not give things like Pepto Bismol or Imodium.
2- No food or water for 6-8 hours, need to give their gastrointestinal tract time to rest. Then, offer a small amount of water followed by a diet of rice (75%) and boiled boneless chicken or ground beef (25%) until the stool is back to normal then wean them back into their regular diet. Keep them indoors in a cool room when fasting.
3- Supplement with a probiotic, these are friendly bacterias that help with regularity. You can buy this over the counter in any pet store.
4- If after doing the above you do not see improvement within 24 hours I would have them checked by your family veterinarian for a hands on examination. If you notice restlessness, abdominal pain, pale gums, etc. have them checked by ER veterinarian.
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