During sexual arousal, the penis can take on a reddish, almost purple color because of the increased blood flow to its blood vessels and glands. But there are other potentially serious reasons why your penis might turn a different color.

The causes of penile discoloration can range from a zipper-inflicted bruise to penile cancer, a rare condition that represents less than 1 percent of cancers in U.S. men. In some cases, a change in penis color is a sign of a sexually transmitted disease (STD) or a noncontagious, harmless skin condition.

Penile discoloration is almost always a cause for a medical evaluation. It may end up being temporary and relatively benign, but you shouldn’t make that diagnosis on your own.

If the change in color is due to an underlying cause that’s medically serious, getting a diagnosis and starting treatment earlier is always a smart approach.

Read on to learn about the possible causes and treatment options for a discoloration of the penis.

A bruise anywhere on the body is the result of tiny blood vessels rupturing just below the surface of the skin. A bruise can form on the penis from a zipper accident, vigorous intercourse or masturbation, or if pinched or struck.

A mild bruise will turn a darker color as it heals and then will fade away. No medical attention may be needed.

A more severe bruise, however, should be evaluated by a doctor. The same is true if a small bruise fails to heal on its own.

To avoid an injury to your penis, be sure to wear protective gear when playing sports and use care when zipping up your pants.

A harmless skin condition, penile melanosis, causes small patches of darkened skin to appear on the shaft or head of your penis, also called the glans. It occurs when melanin forms in concentrated deposits on the surface of your skin.

Penile melanosis isn’t an STD and isn’t contagious.

It’s not well understood why some men develop this skin condition, though there’s some evidence that psoriasis treatment which includes the medication psoralen and ultraviolet light may raise the risk of penile melanosis.

Treatment usually isn’t necessary, though surgical removal of the patches may be possible in some cases. The procedure may leave noticeable scars, however.

Contact dermatitis is a skin reaction to contact with an irritant. Certain soaps or detergents can trigger a reaction anywhere on the body.

When contact dermatitis affects the penis, it’s often caused by a latex condom. You may not know you have a latex allergy until you experience a reaction. The skin can turn red and itchy. A serious reaction can also cause a break in the skin and the release of serum.

Mild cases can often be treated by over-the-counter corticosteroid cream. If there’s a break in the skin, see your doctor to help prevent an infection from developing. Nonlatex condoms are available if you have a latex allergy.

Other possible causes for contact dermatitis are allergies to soaps or detergents.

White patches that form on the penis could be a sign of lichen sclerosus. It’s a skin condition that’s more common in those who aren’t circumcised.

In addition to white spots that grow larger as the disease progresses, the skin of the penis can also become itchy and more fragile. Other symptoms include painful erections and decreased urine flow.

Treatment usually includes a strong steroid ointment applied directly to the affected skin. If only the foreskin is affected, circumcision may be advised.

Lichen sclerosus is a lifelong condition that can go through periods of remission and flare-ups.

It’s not always clear why some men develop it. A family history of the condition may increase your risk. It may also be caused by an autoimmune disorder, which means the body’s immune system mistakenly attacks healthy cells.

If you have lichen sclerosus, you may be at a higher risk for other autoimmune disorders, such as thyroid disease or vitiligo.

Vitiligo is a condition that results when skin cells stop producing melanin, the pigment that gives skin its normal color. Vitiligo typically affects areas such as the elbows and knees, but it can develop anywhere on the body, including the penis. It’s not painful or contagious. The texture of the skin should be unaffected.

Vitiligo may appear as a small whitish spot or may cover a much larger area. There are some steroid treatments that may help, and some medications that work on the immune system may help restore color in small or mildly affected areas.

Syphilis is an STD that can ultimately affect the brain, heart and other organs if not treated early and effectively.

The first sign is usually a white or red ulcer on the penis. It’s usually painless at first. If left untreated, however, an itchy rash can appear on much of the penis and the body. Other symptoms can include headache, fever, and fatigue.

Because syphilis is a bacterial infection, strong doses of antibiotics, such as penicillin, are needed to treat the infection. However, you can be reinfected if you have unprotected sex with someone who has syphilis.

Other STDs, such a genital warts, can cause bumps, growths, and other appearance changes. Getting tested regularly for STDs is helpful in getting early treatment if a test comes back positive. It’s also important to keep from spreading a disease. Practicing safe sex will also help improve your chances of avoiding STDs.

Though cancer of the penis is rare, it’s important to know the signs so you can respond promptly.

One of first symptoms of penile cancer which occurs in an early stage of the disease is a change in color. The shaft or the glans may be affected. The penis may turn reddish, or flat brown patches may form. The skin of the penis itself may also thicken, and the penis may feel sore.

Treatment options include radiation therapy or surgery to remove small sections of cancer from the skin’s surface. The nature of the cancer and how much or little it has spread will help determine what treatment approach is the best.

If you notice any penile discoloration that’s anything other than a mild bruise of which you know the origin, you should see your doctor or a urologist. A urologist is a physician who specializes in the health of the urinary tract and male reproductive organs.

Diagnosing your condition will usually involve a physical examination by a doctor and a review of your medical history and any other symptoms.

Depending on what your doctor suspects is the underlying cause of your penile discoloration, blood tests and other tests may be necessary.

A standard test includes a complete blood count. This test includes a check on levels of:

  • white blood cells
  • red blood cells
  • platelets

High white blood cell levels, for example, suggest the body is fighting an infection. A blood test can also be used to check for STDs, such as syphilis, HIV, herpes, and hepatitis. Other markers for cancer may also be detected in blood test.

Suspicious growths or sores on the penis may be biopsied, meaning a small piece of tissue is carefully removed and studied under a microscope. This can sometimes reveal cancerous cells or signs of other disease.

Any change to the appearance of your penis, especially discoloration or the formation of unusual patches or growths, can be alarming. Don’t hesitate to seek medical attention. Early detection and treatment may help reduce your risk for other health complications that can come from an untreated condition.

If a change in color occurs that’s harmless but permanent, consider talking with a therapist or other mental health professional to help you adjust to the change.