An early study5 revealed onion toxicity in dogs when the amount of onions fed was more than 0.5% of the animal’s weight. To induce hemolytic anemia, dogs in one study6 were fed 30 g of onions/kg of body weight (which is 3% of body weight) once daily for 3 days. In another study,4 dogs ranging in weight from 9.9 to 12 kg were fed 200 g one time (1.6% to 2% of body weight). Depending on the size and type, an onion’s weight can range from 0.5 to 16 oz (14 to 455 g). A review of the ASPCA Animal Poison Control Center case record database for Allium cepa exposure over a 2-year period revealed 23 cases (20 dogs, 3 cats). Six of the affected dogs and one cat showed clinical signs. The data revealed that it takes a fairly large amount of ingested onions before signs are observed but that the dose in the reported cases was consistently over 0.5% of the animal’s body weight.

For cases in which a significant amount of onions have been ingested
within the past 2 hours and there are no contraindications to emesis (i.e.,
hypoxic, comatose, dyspneic, lacking normal pharyngeal and/or gag reflexes), vomiting should be induced by giving the animal 3% hydrogen peroxide orally (1 ml/lb of body weight, not to exceed 45 ml), followed by the administration of activated charcoal.

So Tracy, for my future reference, how DO you make an 80 pound bulldog vomit? (picturing a wrestling match - bruises and abrasions...)