A dog collar needs proper fit -- neither too loose or nor too tight -- for the dog's comfort and safety. If you notice fur loss under the collar, it's natural that you're concerned for your puppy's health. Once a vet has ruled out medical reasons for the hair loss, you can take specific steps to prevent baldness in the area.
Ensure Proper Fit
As a general rule, you should be able to easily slip two fingers between the collar and the dog's fur. More room than this indicates the collar is too loose; less room indicates it is too tight -- which can lead to hair loss over time, not to mention is a choking hazard. Some dogs get their hair caught in ill-fitting collars, which can cause it to be pulled out.
Change Collar Material
Collars are made of a variety of materials, some of which may cause allergic reaction and others whose textures cause snagging or friction. If your vet indicates your dog may have allergies, purchasing a collar of a different material may prevent future hair loss. Try a variety of styles if allergies aren't a cause.