Seborrheic keratoses are common, harmless, noncancerous lesions or growths on the skin. They usually appear as tan, brown, yellow, gray, or very dark brown warty growths ‘stuck onto’ the back, shoulders, chest, or face.

Also known as basal cell papilloma or seborrheic warts, seborrheic keratoses can appear anywhere on the skin except the palms, soles, and mucous membranes. They often appear on the scalp, trunk, neck, arms, and legs and sometimes occur on the face. Some look black but are, in fact, very dark brown.

Seborrheic keratoses can look like warts but are different from warts. Warts result from a virus, such as the human papillomavirus (HPV). Seborrheic keratoses are unlikely to stem from HPV or another virus, according to Dermnet NZ. They are not contagious.

Seborrheic keratoses tend to appear from middle age onwards. Some individuals may have just one, but there will usually be several. Seborrheic keratosis is not contagious.

Over 80 million people in the United States have seborrheic keratoses. They can occur at any age but are most common after the age of 50 years.

Seborrheic keratoses may look like warts, moles, or skin cancer.

They can have the following features:

  • Color: Tan, brown, yellow, gray, or skin-colored; some are very dark brown but may appear black.
  • Location: Commonly on the trunk, scalp, trunk, neck, limbs, and sometimes the face. They do not affect the soles of the feet or palms of the hands.
  • Texture: Can be “waxy” in appearance or rough and crusty, like a barnacle.
  • Raised or not: Can look as if “stuck onto” the skin, but some are flat.
  • Size: From a pinpoint to over 1 inch (around 2 millimeters to 3 centimeters) across.
  • Shape: Usually round or oval.
  • Sensation: Not painful but may itch or become irritated.
  • Number: Ranging from one to hundreds of lesions.
  • Growth: Lesions usually start as small, rough bumps and gradually get thicker and develop a warty surface. They grow slowly.

Some lesions can be light-colored and flat, like a solar lentigo, which is also known as a sunspot or “liver” spot.

On darker skin, seborrheic keratoses present similarly but are more likely to be the darker brown type.

Seborrheic keratosis lesions are rarely painful but can be annoying, depending on their position. It is important not to pick or scratch them as this can lead to bleeding, swelling, and potentially, infection.

There are many types and subtypes of seborrheic keratoses, including stucco keratoses and dermatosis papulosa nigra.

Stucco keratoses are multiple small, light tan papules or bumps with a rough texture, specifically around the lower legs and ankles.

Dermatosis papulosa nigra appears as multiple small, dark pigmented papules or bumps. Lesions often appear on the face. They tend to affect people with brown or black skin.

It can be difficult to distinguish between seborrheic keratoses and skin cancer lesions. For this reason, anyone who notices new lesions or skin changes should see a doctor.

If they already have a diagnosis, they should request an appointment if the following occur:

  • a large number of growths grow in a short space of time
  • the borders around the growths are irregular or blurred
  • the growths are irritated by clothes and regularly bleed, as removal may be an option
  • growths or sores develop and do not heal
  • the growths are an unusual color, for instance, purple, reddish-black, or blue
  • there is itching or irritation

People with multiple seborrheic keratoses may wish to make a yearly appointment with a dermatologist to check for changes that could be cancerous.

How do you know if a lesion is cancerous?

In most cases of seborrheic keratosis, no further treatment is necessary.

However, a doctor may recommend removing the growth in the following cases:

  • It is hard to distinguish from skin cancer or the diagnosis is uncertain.
  • The individual does not like it and wants to remove it.
  • It causes itching or irritation, or clothing and jewelry rub against it.

If the dermatologist carries out a biopsy, they will usually remove the seborrheic keratosis at the same time.

There are several ways to remove seborrheic keratoses.

Cryosurgery

A doctor applies liquid nitrogen to the growth with a spray gun or cotton swab. The lesion freezes and falls off in a few days. A blister may form that will eventually dry into a crust and fall off.

However, the lesion can grow back, and changes in pigmentation can also occur.

How can cryosurgery treat a wart?

Skin shaving (shave excision)

A doctor shaves off the lesion with a scalpel without touching the layers beneath the skin’s surface. The collected cells can then go to a laboratory for a biopsy.

Electrocautery, curettage, or both

In electrocautery — also known as electrosurgery — the surgeon uses an electric current to burn (cauterize) the growth, under a local anesthetic. In curettage, a doctor uses a curette — a scoop-shaped surgical instrument — to scrape off the burnt growth.

A doctor will usually apply a local anesthetic to numb the area. On the face, however, they will use a lower current and will not numb the area.

Some people need only electrocautery, some need curettage, and others need both.

Laser therapy

The doctor uses a laser to remove the lesion.

Laser treatment can be ablative or nonablative. Ablative surgery removes the top layer of skin and heats the underlying layer to stimulate the growth of new collagen fibers. Nonablative surgery is less invasive and works by stimulating the growth of new collagen.

Topical treatments

Researchers are currently looking into options for topical applications to remove seborrheic keratoses.

The Food and Drug Administration (FDA) has approved a 40% hydrogen peroxide solution for raised keratoses that appears safe and effective. Minor adverse effects may occur, such as scaling, inflammation, and changes in skin color.

Chemical peels are also available. A doctor applies a peel containing trichloracetic acid to the lesion.

All these methods have disadvantages, such as:

  • irritation and discomfort in the area during recovery time
  • changes in pigmentation, where the skin becomes lighter or darker
  • the need for several sessions, as a doctor may be unable to remove all lesions in one intervention.
  • recurrence of growths or lesions continue to appear in other areas

There are no proven home remedies for seborrheic keratoses.

Lemon juice or vinegar can irritate the skin, possibly causing the lesion to dry and crumble. However, there is no evidence that this is safe or effective.

Some substances, such as tea tree oil, can trigger allergic contact dermatitis, an immune system reaction that causes itching and inflammation around the site.

Anyone with concerns about a lesion should see a doctor, who can check that it is not melanoma and advise on treatment, if appropriate.

Dermatologists are not certain why seborrheic keratoses develop.

Possible causes and risk factors include:

  • Skin damage: Some types of seborrheic keratoses appear after dermatitis or sunburn.
  • Genetic factors: Scientists believe a genetic mutation could be responsible when people develop a high number of seborrheic keratoses or when they run in families.
  • Age: They are more common in those aged 50 years or above.
  • Exposure to ultraviolet (UV) light: Similarities with solar lentigo suggest UV light may play a role.
  • Medication: People who use epidermal growth factor receptor inhibitors to treat cancer may have a higher risk of warty growths.
  • Skin friction: This may increase the risk and explain why they often appear in skin folds.

Seborrheic keratoses do not appear to be linked to any viral infection.

The authors of a 2016 research article note that seborrheic keratoses have “virtually no malignant potential.”

In rare cases, however, a sudden onset of numerous seborrheic keratoses may be linked to underlying malignancy, such as cancer in the gastrointestinal tract, lymphoma, or leukemia. In this case, it is known as Leser-Treélat sign.

Sudden eruptions of numerous seborrheic keratoses can also occur in pregnancy, during chemotherapy, and with various inflammatory skin disorders.

To diagnose seborrheic keratosis, a physician or dermatologist:

  • will ask about symptoms and family and medical history
  • will carry out a visual and physical examination
  • may recommend a biopsy to rule out skin cancer and remove a growth at the same time
  • may recommend further tests to rule out other conditions

A lesion with the features of seborrheic keratosis is unlikely to be skin cancer.

However, since it can be hard to distinguish between skin cancer and seborrheic keratoses, it is best to check with a doctor, especially if multiple lesions appear in a short time or if lesions change.

If the doctor suspects melanoma or is unsure, they may recommend a biopsy for examination under a microscope.

Seborrheic keratoses rarely become cancerous, but they share genetic features with some types of cancer. Researchers believe the gene process used by the lesions could help scientists learn more about certain types of cancer.

Here are some answers to questions people often ask about seborrheic keratoses.

How do you get rid of seborrheic keratoses?

In most cases, seborrheic keratoses do not need treatment, but a person may opt to remove them if they snag on clothing or feel unsightly. A doctor may remove one while taking a biopsy for skin cancer. Options include minor surgery, laser therapy, and cryotherapy. A doctor can advise on the best approach.

What do seborrheic keratoses look like?

Seborrheic keratoses can be skin-colored, tan, brown, yellow, gray, or very dark brown that may appear black. They can look waxy or rough, like a wart. Some are raised, as if stuck onto the skin, while others are flat. The size can range from a pinpoint to over 1 inch (around 2 millimeters to 3 centimeters) across. They usually grow slowly.

Are seborrheic keratoses serious?

Seborrheic keratoses are not cancerous but can share some features with malignant lesions. They can be hard to tell apart from skin cancer, so it is essential to see a doctor about any new or unexplained lesions.

Seborrheic keratoses are harmless lesions that appear on the skin, especially as people get older. They can occur anywhere on the body except the soles of the feet and palms of the hands.

They may be yellow, brown, tan, or skin-colored. Some look black but are, in fact, dark brown. They may be raised as if stuck onto the skin, but some are flat.

It can be hard to tell the difference between seborrheic keratoses and skin cancer. For this reason, a person should see a doctor if they notice a new lesion or if a lesion changes in any way.