Bruising on the arm may be related to trauma, medication side effects, blood disorders and other reasons. 1. Trauma: If the arm is subjected to extrusion, impact and other traumatic injuries, it can cause rupture of subcutaneous blood vessels and bruising. 2. Drug side effects: such as aspirin and other antiplatelet drugs, its common side effects include spontaneous bleeding, which can be manifested as bruises on the arm. 3. Blood diseases: such as thrombocytopenic purpura, etc., due to the reduction of platelets, can cause subcutaneous bleeding on the arm, which can be manifested as bruises. It is recommended that people with bruises on the arm go to the hospital in time to clarify the cause of the disease and standardize the treatment under the guidance of the doctor to avoid delaying the condition.