Dog collars should not cause your pet discomfort or hair loss. If your dog is losing hair because of his collar, it is a sign that something is wrong. Work with your veterinarian to determine what underlying problem is responsible for the hair loss and how to correct the condition so the hair can grow back.
Your dog's collar should fit comfortably but tightly around your dog's neck. It should not be loose enough to slip off or so tight that it chokes your dog. You should be able to fit two fingers comfortably between the collar and your dog's neck. Check and adjust the collar periodically to make sure it fits well as your dog grows, gains or even loses weight.
If your dog's collar is too tight, it is possible that it will cause hair loss or even cut into the skin creating a wound. If you believe your dog's hair loss is the result of a collar being too tight, remove the collar immediately and contact your veterinarian.
Your dog can be allergic to his collar, especially if it contains any type of medication. Flea and tick collars may have this effect on some dogs because of the chemicals they contain. This condition is called a contact allergy and can cause itching, scratching, irritated skin and hair loss. Removing the offending item, in this case the dog's collar, should eliminate the problem. Note that it may take time for your dog's hair to grow back.
An assortment of medical conditions can lead to canine hair loss. While the area under the collar initially might appear to be the only area affected by hair loss, that doesn't mean it will stay the only area affected. If your dog is losing his hair and you cannot figure out why, get him to the veterinarian immediately. Conditions such as mange will not improve on their own without treatment and tend to get worse over time.