Tuna (canned) — Food safety facts
Feed with care · Dogs · full breakdown on the linked food page
Occasional small amounts in water (not oil, not seasoned) are okay. Mercury builds up — do not feed regularly, especially to cats.
Dogs and cats carry the same rating (Feed with care), so the notes below hold for either pet.
Tuna (canned) is not poisonous to dogs, but it is easy to over-feed. Rich fat, salt or seasoning can bring on vomiting or diarrhoea within a day, and repeated large portions are a recognised trigger for pancreatitis.
If it ate some before you noticed, watch for vomiting, diarrhoea, loss of appetite, unusual tiredness or a painful, swollen abdomen over the next 24 hours. A one-off small amount in a healthy adult usually passes with water and rest; repeated exposure, a puppy or kitten, a senior, or a pet with an ongoing illness is a reason to call your vet the same day.
Tuna (canned) can turn up now and then, but not as a daily habit: keep the rest of the day's food unchanged and watch the next walk and the stool that comes with it.
The full verdict, portion guidance and serving notes are collected on the Tuna (canned) food facts page.