You notice some blood, dog licking at a paw, limping, or yelping...then find that ghastly torn nail. Ouch. Or, you may find blood everywhere.
Active bleeding or not, that torn nail must be attended to.
A Word of Caution
Every dog is different—you know your dog best, but remember that when injured, a dog will instinctively try to protect himself. This means that your dog may reflexively bite or snarl at you, not really "meaning" to.
It is wise to use a muzzle.
If possible, remove any of the broken parts of the nail that may still be attached. This broken end often causes the dog more pain and may increase or continue the bleeding every time the torn piece is disturbed.
The quickest way to do this is with a dog toenail clipper. Sometimes the piece is barely hanging on and they can be pulled off (quickly) with your hand.
The injured nail needs to be gently washed off. Warm water to remove any debris lodged between the nail and the toe or leg.
If there is active bleeding, wait on washing. Apply gentle firm pressure with a clean cloth to the area. A firm grasp around the entire foot works best if the dog will allow it.
Often this type of injury leaves a bloody "stump" of tissue that would normally be inside the toenail housing. This is very tender and sensitive.
It is recommended to have your vet take a look at this type of injury as soon as possible.
If a large amount of toenail has been removed, most vets will bandage the area and prescribe a short course of antibiotics as prophylaxis against infection. Another method is to use an antibiotic ointment (with frequent bandage changes) on the stump for lubrication and reduced friction and pain.
In severe or repeated injury cases, especially for the dewclaw, your vet may recommend removal of the toe.
Sometimes a toenail injury happens without any known trauma or reason. A veterinary examination is important to rule out other possible causes, such as an infection or tumor in the area, weakening the toenail and causing secondary breakage.
What to Do If You Cut Your Dog's Toe Nail too Short
It is a good idea to have shaving alum or styptic pencils at home for general first aid—when a nail is accidentally cut too short, you have the necessary tools on hand to stop bleeding. Alum and styptic pencils can be purchased over the counter at drug stores in the first aid supply area.
If you don't have alum or a styptic pencil, you can use flour or corn starch to help stop bleeding. Pack a small amount in the cut nail end and apply pressure. Holding ice on the cut surface (if the dog will allow) will also help stop bleeding.
For some great pointers on cutting nails, use this photo tutorial on how to trim a dog's nails or this one on how to trim a cat's nails and the tools you will need. Both are provided by Washington State University.