Dogs bite their tails for one of two reasons: either medical or behavioural. Medical reasons for itching can include fleas, a skin disorder or irritation, or impacted anal glands. A skin condition can cause a dog to bite until the flesh is raw, increasing the chance of infection and sometimes requiring antibiotics. Behavioural issues usually stem from boredom or obsessive tendencies. A long wagging tail is a temptation within easy reach. Restless dogs who get bored need the stimulation of exercise or other activity to distract them. Before you treat your dog, determine why he bites his tail.
Examine your dog's tail carefully for signs of fleas, hot spots, or redness. If a skin issue is involved, see your veterinarian for advice.
If the biting has medical origins, the vet might give you a topical antibiotic for the tail. To make sure your dog leaves his tail alone so the sore can heal, bandage it with medical tape or spray it with bitter apple to make the spot less appealing.
If you treat the problem without veterinary help, be sure whatever substance you put on the tail is nontoxic, since the dog will probably gnaw at it. Try aloe or a lukewarm bag of green tea to soothe the spot or dusting powder to dry it.
If you see no physical reason for him to bite his tail, it's a behavioural issue. Make sure he has plenty of toys to occupy him when you aren't with him.
When he bites, do not offer rewards or treats to divert his attention. Instead offer him a toy or walk away from him. Keep a variety of toys near spots where he spends time.
Distract him from biting by taking him for a walk. Make it a routine that he learns to looks forward to. If possible, exercise him in a large fenced area where it's safe to run, or let him play with other dogs.
If he continues to bite his tail, the problem might require the services of a pet-behaviour specialist. Your veterinarian should also be consulted regarding the next step.
Lubricating your dog's skin will decrease his desire to bite. Give him a teaspoon of oil with each meal and a teaspoon of apple-cider vinegar in his water to repel fleas. Some dogs tolerate having an open-ended sock secured with medical tape over the infected spot so the medicine has a chance to heal the infected area. If the dog persists in biting, an Elizabethan or Bite-Not collar or even a muzzle might be necessary to restrain him. Your vet might also recommend a veterinary specialist.