An ulcer on the penis may be caused by infections such as herpes, syphilis, or other skin infections, and sometimes by friction or a minor injury. Because the exact cause cannot be confirmed without examination, it is important to consult a doctor as soon as possible. You should avoid any sexual activity until you have been examined by a doctor, as this may worsen the condition or pass an infection to a partner. Keep the area clean and dry, and do not apply strong creams or home remedies without guidance, as these may irritate the skin further. A doctor may examine the ulcer and suggest tests such as blood or swab tests to identify the cause. Treatment will depend on the diagnosis and may include antibiotics or antiviral medicines if needed. Early assessment is important to ensure proper treatment and to prevent complications or the spread of infection.