Tetanus shots are certainly painful, but why do they hurt? Firstly, because a skin test is needed for tetanus, which primarily involves injecting the drug under the skin of the forearm to form a small bump about 0.5cm in diameter. It is known that there are abundant nerve endings under the skin, so the injection of the drug can cause local swelling and compress the surrounding nerves, and the drug can also cause an irritation response in the surrounding nerves, which is why tetanus shots are painful.