Because your dog's toenails have live blood vessels in them, cutting them too short can cause him pain and make him bleed. Stay calm -- you can easily stop the bleeding without going to the veterinarian. Your dog may need a few treats to take his mind off of the miscalculation, but once you stop the bleeding and calm him down, he'll be just fine.
Talk to your dog in a calm, gentle voice, because he's probably in pain. Stroke his fur to calm him down and give him a couple of small treats. Once he settles down, apply a clotting powder -- you should always have this on hand before trimming your dog's nails. Apply it directly to the bleeding nail to facilitate clotting and stop the bleeding. In a worst-case scenario, if your dog won't allow you to apply the powder or you find that you're all out, the bleeding should naturally stop within a few minutes. If it doesn't, contact your vet right away.
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