Obvious injuries caused by pulling and jerking are to the trachea and neck. Less obvious are the injuries to nerves and vertebrae which can cause permanent damage as well as lead to other medical issues such as organ damage, paralysis, or neurological issues.
As an example, the collar rests right where the thyroid gland is and repeated pulling on the leash causes pressure on the thyroid, which can become injured. This may ultimately lead to hypothyroidism.
The nerves in the neck that go down to the paws can be damaged when jerking on the leash, manifested by your dog's licking his paws (sometimes misdiagnosed as allergies). Similarly, he can also scratch his "armpits" or his ears. He can get epilepsy due to the compression of jugular vein in the neck, increasing the pressure in the vein and causing a seizure. Moreover, nerves in his neck can become compressed, which can lead to paralysis in his limbs.
Particularly harmful are retractable leashes because when your dog is running and gets to the end of the leash, he gets jerked very quickly with all the force that he was using to run. All of that force goes into his neck.
Be Careful with Choke Chains & Prong Collars
Other collars can cause injuries such as choke chains. They tighten on your dog's neck rather than just putting pressure on it. The theory behind choke chains is that both the sound and the pressure will cause a startle response resulting in your dog stopping pulling. However, most people do not release quick enough, and, compounding that, they do not put the chains on correctly.
Prong collars can be dangerous as well because even though the pressure is distributed around your dog's neck, one of the points can be directly over your dog's larynx or thyroid having the same problems as previously mentioned.
Use Harnesses
When it comes to safety and comfort, using a harness is ALWAYS the best way to walk a dog. Not only does this alleviate any pressure on the neck, it also makes it easier to pull dogs out of harm’s way if they get into trouble.
There are many kinds of harnesses to choose from.