A pizza topping built for people, not dogs
Pepperoni earns a no as a deliberate dog treat, even in the single-slice form that slid off your homemade pizza. It is a cured, fatty sausage seasoned for human taste, which means a concentrated package of fat, salt, and spice in a coin your dog can swallow in one gulp. The seasoning blends used for pepperoni commonly include garlic and onion powders, both of which are harmful to dogs in sufficient quantity, and the paprika and chilli warmth that make it pleasant for people can irritate a dog's stomach. Unlike a plain piece of cooked meat, there is no trimmed, rinsed version of pepperoni that fixes its basic composition.
What one slid-off slice usually does
Context keeps this in proportion. A single pepperoni slice snatched by a healthy medium or large dog is unlikely to cause more than a very thirsty evening and possibly a loose stool the next day, because the garlic and onion in one slice are present in small amounts. The picture changes with quantity and with the dog: several slices, a whole stick of pepperoni, or a raid by a small dog, a puppy, or a dog with a history of pancreas flare-ups can add up to vomiting, stomach pain, and a miserable night. Make sure fresh water is freely available after any pepperoni incident, and do not offer fatty food to soak it up; plain normal meals are the kindest follow-up.
Why pizza night trains the wrong lesson
The bigger cost of the slid-off slice is behavioural. A dog who is handed pepperoni whenever pizza is made learns that the pizza box predicts premium snacks, and will supervise every future pizza night at close range, ready to dive for anything that slides. That hovering is how whole slices, garlic-laden crusts, and dropped toppings in sauce end up eaten at speed. If you want a pizza-night ritual for your dog, give him a small piece of plain cooked chicken or a few of his regular treats on his mat when the pizza arrives, so the routine exists but the topping never becomes his.
When a pepperoni episode needs the vet
Crusts deserve a mention on pizza night too, because homemade crust edges brushed with garlic butter carry the same seasonings as the topping. If your dog has a pizza-night mat spot, settle him there before the box opens, with his own approved treat ready, rather than negotiating once the smell of pepperoni is already in the air and every slice is sliding.
Call your vet if a small dog eats several slices, if any dog raids a quantity of cured meat, or if you see repeated vomiting, a painful or bloated-looking belly, marked lethargy, or diarrhoea that does not settle within a day. Dogs known to be prone to digestive flare-ups after fatty food should be discussed with the vet even after a modest amount if they seem uncomfortable. Pale gums or weakness after eating seasoned cured meats also deserve prompt advice, however unlikely they are after one slice. Recording pepperoni and pizza toppings as never-feed items in KinsLe through Care Teams keeps the rule intact when friends order in, so your dog is not quietly adopted as the topping finisher by someone who does not know better.