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Friend, onion toxicity in dogs is very dependent on the amount of ingestion ingested. So for a 9 kg dog, he would need to ingest at least 46 grams to be concern about toxicity. If he didn't eat the onions highly unlikely for the this to be toxic, no need to be concern he will be fine.
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If he did not eat the onions it will not be toxic. Plus, he would need to eat at least 46 grams to be concern about toxicity.
Please reply if you have additional questions.