How a 5-ounce can affects a 12 kg dog
Six hours in, the fact that your dog shows no signs is reassuring—most digestive upset from a sudden rich meal would have appeared by now. For a 12 kg dog, a whole can is roughly triple the "occasional treat" amount, so his system is working hard to process the protein load. You might notice slightly looser stools tomorrow as his body clears the excess.
Though mercury isn't an immediate concern, this sizeable dose means his liver will be breaking down more than usual. Watch for subtle clues like taking longer naps or being less interested in his next meal. These could indicate his organs are prioritizing detoxification over normal activities. If they persist beyond 24 hours, a vet visit would be wise to check liver enzymes.
Critical monitoring window
The next 48 hours are key for spotting delayed reactions. His stomach acid likely handled the initial load, but the intestines might rebel later with gurgling sounds or gas. Keep his diet bland (boiled chicken and rice) for two meals to ease digestion. Avoid fatty foods that could further challenge his pancreas.
Hydration matters—tuna's salt content may make him drink more. Offer water frequently but in small amounts to prevent gulping-induced vomiting. Check his gums; they should be moist and pink, not tacky. If he skips two meals or seems lethargic when awake, these warrant a vet call even without vomiting.
What blood tests could reveal
If you do visit the vet, they'll likely run a chemistry panel focusing on kidney (BUN, creatinine) and liver (ALT, ALP) values. These show how well his organs processed the tuna's protein and mercury. For a one-time incident, minor elevations might not need treatment, but the results establish a baseline in case he ever gets into tuna again.
A complete blood count (CBC) could detect inflammation from food irritation, though that's less common with fish than with fatty meats. The vet may also check urine concentration—high protein meals sometimes cause temporary changes. None of this is urgent now, but having the records helps if he develops chronic issues later.
Long-term considerations for repeat offenders
Dogs that snatch human food often try it again. Store tuna cans immediately in locked cabinets, not just high shelves. If he's drawn to fish smells, wash dishes with lemon juice to neutralize odors. Train a solid "leave it" using lower-value treats first—his successful theft makes this behavior self-rewarding.
For future safety, know that oil-packed tuna poses additional risks. The fat could trigger pancreatitis, especially in small dogs. Water-packed is always safer, but no tuna should become a habit. If he seems obsessed with fish, ask your vet about omega-3 supplements formulated for dogs—these provide benefits without mercury risks.
The full verdict, portion guidance and serving notes are collected on the Tuna (canned) food facts page.