Swollen and bleeding nipples when boys develop may be caused by allergies, breast diseases, trauma or friction. 1. Allergy: When boys develop, if the nipple comes into contact with allergens, redness, swelling, pain, itching and other symptoms may occur, and when the itching is severe, bleeding may occur. 2. Breast disease: when the hormone level in the boy’s body is abnormal, breast disease may also occur, such as breast cancer, breast hyperplasia, etc. When the disease appears lesions, it may lead to nipple enlargement and bleeding phenomenon. 3. Trauma or friction: When boys develop, excessive exercise or clothing friction will easily lead to nipple injury, nipple rupture, resulting in nipple enlargement and bleeding. Boys should go to the hospital in time to see the cause of nipple enlargement and bleeding, and then give appropriate treatment under the guidance of the doctor, so as to avoid delaying the condition.