In dermatology, SK stands for seborrheic keratosis, a common noncancerous skin growth that appears as a waxy, slightly raised spot on the skin’s surface. These growths are harmless, not contagious, and not considered a risk factor for skin cancer. They’re so common that studies show virtually 100% of people over age 50 have at least one.
What Seborrheic Keratoses Look and Feel Like
The hallmark of an SK is its “stuck-on” appearance. It looks like someone pressed a small blob of candle wax onto the skin. The surface is often rough or bumpy, with a texture that can feel scaly or slightly greasy. Colors range widely: light tan, dark brown, or nearly black. A single person can have growths of different colors and sizes at the same time.
SKs tend to show up on the chest, back, shoulders, face, and scalp. They typically spare the palms and soles. Most people develop more of them as they age. Starting as small, flat spots, they can gradually thicken and enlarge over months or years, sometimes reaching a centimeter or more across. They often develop tiny crater-like openings across their surface, a feature dermatologists use to help identify them.
Why They Develop
SKs result from an overgrowth of ordinary skin cells in the outer layer of skin. The exact trigger isn’t fully understood, but genetics play a clear role. Between 30% and 70% of seborrheic keratoses carry a specific mutation in a gene called FGFR3, which controls cell growth. These mutations aren’t inherited from your parents. They develop spontaneously in individual skin cells over time, possibly from cumulative ultraviolet light exposure. The altered gene produces an overactive protein that drives skin cells to multiply more than they should, forming the raised bump.
Age is the biggest risk factor. Prevalence climbs steadily from about 12% of people aged 15 to 25 up to 100% of those over 50. Family history matters too. If your parents had many SKs, you’re more likely to develop them in large numbers.
How Doctors Tell SKs From Skin Cancer
Because darker seborrheic keratoses can look similar to melanoma or other pigmented skin cancers, dermatologists rely on close visual inspection, often with a handheld magnifying tool called a dermoscope. Under magnification, SKs reveal characteristic features that melanoma almost never produces: small white cyst-like dots (called milia-like cysts) and tiny dark pore-like openings (comedo-like openings). In a study comparing tricky cases, the combination of these features appeared in 56% of seborrheic keratoses and in zero melanomas.
Melanoma, by contrast, tends to show irregular color patterns, asymmetric borders, and certain structural features like shiny white lines visible under polarized light. If a growth changes rapidly, bleeds without clear cause, or looks unusual even for an SK, a dermatologist may take a small biopsy to examine it under a microscope and rule out anything concerning.
When Multiple SKs Appear Suddenly
A rare but important exception to the “SKs are harmless” rule is something called the sign of Leser-Trélat: the abrupt eruption of many seborrheic keratoses, or a sudden increase in the size and number of existing ones. This is considered a paraneoplastic syndrome, meaning the skin changes are driven by an underlying internal cancer rather than the normal aging process. The associated cancers tend to be aggressive, and the average survival after diagnosis is about 10.6 months.
This scenario is uncommon. Gradually accumulating a few new SKs each year is normal. What raises concern is dozens appearing within weeks, especially alongside unexplained weight loss, fatigue, or other systemic symptoms.
Removal Options
Because SKs are benign, removal is cosmetic or comfort-based. Insurance rarely covers it unless a growth is irritated by clothing, constantly catches on jewelry, or needs a biopsy to confirm the diagnosis. The three standard in-office methods are:
- Cryotherapy: Freezing the growth with liquid nitrogen. It works well on thinner, flatter spots but is less effective on thicker ones. On darker skin tones, it carries a risk of permanent pigment loss in the treated area.
- Curettage (scraping): After numbing the skin, the dermatologist uses a blade to scrape the growth off the surface. This is sometimes combined with cryotherapy for flatter lesions.
- Electrocautery (burning): The growth is destroyed with a small electrical current after numbing. This method is often paired with scraping for thicker growths.
All three methods can cause temporary or permanent changes in skin color at the treatment site, and minor scarring is possible. Healing typically takes one to two weeks for small lesions.
Topical Treatment
A high-concentration hydrogen peroxide solution (40%) is the only FDA-approved topical treatment for raised seborrheic keratoses. Applied in a dermatologist’s office (not at home), the solution is dabbed directly onto each growth during one or two visits spaced about three weeks apart. In clinical trials involving over 900 patients, about 30% of treated lesions cleared completely and roughly 51% were clear or nearly clear, compared to just 7% with a placebo. The results are modest compared to scraping or freezing, but it’s a non-invasive alternative for people who want to avoid procedures.
Removed SKs do not grow back in the same spot, but new ones can develop nearby over time. There’s no proven way to prevent seborrheic keratoses from forming, though limiting UV exposure may slow the accumulation of the genetic mutations that drive them.