“Barnacles” on human skin is an informal nickname dermatologists and patients use for seborrheic keratoses, the single most common benign skin tumor in adults. These waxy, slightly raised, sometimes rough growths tend to multiply with age and can look like they are stuck onto the skin’s surface, which is how the marine analogy stuck. They are not caused by an infection, they are not contagious, and in the vast majority of cases they are completely harmless. Yet they create genuine confusion because some of them can closely resemble skin cancers, and their sudden appearance in large numbers can occasionally signal something more serious happening inside the body.
What Seborrheic Keratoses Actually Look Like
Seborrheic keratoses (SKs) range widely in appearance, which is part of what makes the nickname “barnacles” so apt and so misleading at the same time. They can be flat and barely noticeable or thick and lumpy. Color ranges from light tan to dark brown to nearly black, and their surface often has a waxy, crumbly, or “pasted-on” texture that you can sometimes catch a fingernail under. They can appear virtually anywhere on the body except the palms and soles, but the face and upper trunk are the most common locations.1Wiley Online Library / JDDG. Seborrheic keratosis Some people develop a handful over their lifetime; others end up with dozens or even hundreds.
The growths come in several subtypes based on how the cells arrange themselves under a microscope, but from the outside the differences that matter to you are practical ones. Some SKs are smooth and flat, easy to mistake for a large freckle. Others develop a rough, horn-like projection of hardened keratin on top. The darkly pigmented versions are the ones most likely to trigger alarm, because they can look remarkably similar to melanoma. Under magnification, though, SKs tend to show characteristic surface patterns: tiny cyst-like white dots (called milia-like cysts) and dark pore-like openings (comedo-like openings) embedded in the surface. In one clinical study, comedo-like openings were the most frequently observed feature, appearing in about two-thirds of cases, followed closely by fissure-and-ridge patterns.2PubMed Central. A Clinicopathological and Dermoscopic Correlation of Seborrheic Keratosis
Why They Develop
Age is the single strongest predictor. SKs are rare in young adults and become progressively more common after age 40. By the time people reach their 60s and 70s, having at least a few is the norm rather than the exception. The growths are driven by genetic changes that happen inside individual skin cells over a lifetime. In a study analyzing 65 seborrheic keratoses, activating mutations in a gene called FGFR3 were found in roughly 57% of them, and these mutations were already present in flat, early-stage lesions, suggesting they occur at the very beginning of the growth process.3PubMed. FGFR3 mutations in seborrheic keratoses are already present in flat lesions and associated with age and localization A second gene, PIK3CA, sometimes shows up mutated alongside FGFR3, though less frequently.
These are somatic mutations, meaning they arise spontaneously in skin cells rather than being inherited from your parents. That said, genetics clearly plays a background role. Some families develop large numbers of SKs at unusually young ages. Researchers studying one such family across two generations found FGFR3 and PIK3CA mutations in the family members’ growths, just as in sporadic cases, but none of those mutations were present in the family members’ germline DNA. That means whatever is inherited is not the mutation itself but some underlying susceptibility that makes the skin more prone to acquiring those mutations.4PubMed. Somatic FGFR3 and PIK3CA mutations are present in familial seborrhoeic keratoses If your parents were covered in “barnacles,” you are at higher risk of developing them yourself, even though the exact hereditary mechanism remains unclear.
Sunlight is a trickier factor. SKs show up on sun-exposed and sun-protected skin alike, which has led to debate about whether ultraviolet radiation matters much. There is some evidence that a particular subtype, the adenoid type, turns up more often on sun-exposed areas.5PubMed Central. Clinical and Histopathological Investigation of Seborrheic Keratosis But a study looking at vitamin D levels and cumulative sun exposure in people with SKs found no significant correlation between those measures and the size of the growths.6PubMed Central. Correlation Among Serum Calcidiol, Sun Index, and Vitamin D Intake in Individuals With Seborrheic Keratoses Living in Coastal Area The honest summary is that UV exposure may nudge certain subtypes along but is not the dominant driver the way it is for actinic keratoses or most skin cancers.
When a “Barnacle” Might Not Be a Barnacle
The real clinical danger with SKs is not the growths themselves. It is that a melanoma or other skin cancer can hide behind the same appearance. Melanomas sometimes develop features that mimic SKs: a scaly surface, yellowish keratin, even comedo-like openings. In a study of 134 melanomas that initially looked like seborrheic keratoses, roughly 18% still appeared consistent with an SK even after examination with a dermatoscope, the handheld magnifying tool dermatologists use to evaluate pigmented lesions.7JAMA Dermatology. Dermoscopic Clues for Diagnosing Melanomas That Resemble Seborrheic Keratosis That is a meaningful miss rate, and it is the reason dermatologists advise against assuming that every dark, bumpy growth is harmless just because it “looks like a barnacle.”
On the flip side, most true SKs display a combination of features that help an experienced clinician tell them apart from melanoma. A retrospective study comparing SK and melanoma lesions that both showed a suspicious blue-white veil pattern found that the combination of milia-like cysts or comedo-like openings within that pattern occurred in more than half of the SKs and in none of the melanomas.8PubMed Central. Differentiating Seborrheic Keratosis from Melanoma Among Lesions Exhibiting Blue-White Veil: A Retrospective Study The typical dermoscopic fingerprint of a seborrheic keratosis includes fissures and ridges, hairpin blood vessels with a surrounding white halo, and those comedo-like openings and milia-like cysts already mentioned.9PubMed. Dermoscopy-pathology relationship in seborrheic keratosis When those features are clearly present, a biopsy is usually unnecessary. When they are absent or mixed with features more characteristic of melanoma, a biopsy is warranted.
SKs are not the only benign growths that get confused with something more worrying. They can also resemble actinic keratoses, which are rough, scaly patches caused by cumulative sun damage that carry a small risk of progressing to squamous cell carcinoma. The two conditions can look strikingly similar to the untrained eye, though their biology is quite different. And occasionally, an SK can develop a cutaneous horn, a hard, cone-shaped projection of keratin. Cutaneous horns can arise from a range of underlying conditions; in a large retrospective study of 643 horns, about 39% were found to sit on top of malignant or premalignant tissue.10PubMed. A histopathological study of 643 cutaneous horns So if a growth develops a hard, projecting horn, getting it checked is especially worthwhile.
The Leser-Trélat Sign
One scenario involving seborrheic keratoses genuinely deserves attention. The Leser-Trélat sign refers to the sudden eruption of many new SKs, or a rapid increase in the size and number of existing ones, and it has been associated with underlying internal cancers.11PubMed. The Leser-Trélat sign is associated with nasopharyngeal carcinoma: case report and review of cases reported in China This is rare and remains somewhat controversial in dermatology, because SKs are so common in the age groups most likely to develop cancer that coincidental overlap is expected. Still, the pattern has been reported alongside cancers of the gastrointestinal tract, lung, breast, and nasopharynx, among others. The key clinical feature is the word “sudden.” A person who gradually develops a dozen new SKs over five years is experiencing normal aging. A person who sprouts 30 or 40 in a matter of weeks, especially with accompanying symptoms like unexplained weight loss or fatigue, may warrant investigation for an occult malignancy.
Getting Them Removed
Because SKs are benign, removal is almost always elective. Insurance in many countries will not cover it unless the growth is irritated, bleeding, or diagnostically uncertain. But cosmetic motivation is legitimate, and the options have gotten better over the years.
The traditional workhorse methods are cryotherapy (freezing with liquid nitrogen) and curettage (scraping the growth off with a blade-like instrument after numbing the area). A head-to-head trial found that both produce highly satisfactory cosmetic results over time, with patients rating their skin appearance around 9 out of 10 by the time a year had passed.12JAMA Dermatology. Effectiveness of Cryosurgery vs Curettage in the Treatment of Seborrheic Keratoses A more recent prospective study comparing curettage, a 532-nm laser, and cryotherapy found curettage achieved the highest lesion clearance rate at about 88%, compared with 55% for laser and 50% for cryotherapy. Patients perceived the laser’s healing process most favorably, however, and cryotherapy produced more side effects than the other two methods.13PubMed. Practical Approaches for Seborrheic Keratosis Treatment: Curettage Versus 532-nm Lithium Borate Laser Versus Cryotherapy: A Prospective Interventional Study
A newer option is topical hydrogen peroxide at prescription-strength concentrations. A 40% hydrogen peroxide solution applied in-office (this is not the drugstore bottle) cleared facial SKs in about 65% of cases after up to two treatments, compared with roughly 10% for the vehicle alone. It was less effective on the trunk and limbs, clearing about 46% and 38% respectively.14PubMed. Anatomic Site-Specific Treatment Response With 40% Hydrogen Peroxide (w/w) Topical Formulation for Raised Seborrheic Keratoses: Pooled Analysis of Data from Two Phase 3 Studies A study using a 30% concentration reported about half of treated lesions reaching complete clearance after one session, with no scarring or severe side effects.15PubMed. Utility of 30% hydrogen peroxide in the treatment of seborrheic keratosis The appeal of hydrogen peroxide treatment is that it avoids cutting or freezing and generally heals without a scar, but the trade-off is a lower single-session clearance rate, meaning you may need repeat visits.
A word of caution about at-home removal. The internet is full of suggestions involving apple cider vinegar, tea tree oil, and drugstore-strength hydrogen peroxide for dissolving SKs at home. These approaches are unregulated, and none has been evaluated in controlled studies the way in-office treatments have. The risk is not just that they will not work. The bigger risk is that you are treating something you have diagnosed yourself. If the growth turns out to be a melanoma or squamous cell carcinoma rather than an SK, self-treating it at home can delay a diagnosis that matters. Any growth you are uncertain about deserves a professional evaluation before you try to remove it.
How Dermatologists Tell Them Apart From Skin Cancer
If you go in for a check, the evaluation typically starts with a dermatoscope, essentially a handheld lens with polarized light that reveals structures invisible to the naked eye. Experienced dermatologists can identify most SKs confidently this way. Research into melanomas that mimic SKs has identified dermoscopic red flags to look for: a pigment network, blue-white veil, irregular dots and globules, and streak-like structures all push the diagnosis toward melanoma even when the surface looks warty or keratotic.7JAMA Dermatology. Dermoscopic Clues for Diagnosing Melanomas That Resemble Seborrheic Keratosis Research is also underway into artificial intelligence tools that can distinguish SKs from actinic keratoses using clinical photographs, and early results show strong performance.16PubMed Central. Artificial Intelligence-Based Distinction of Actinic Keratosis and Seborrheic Keratosis In the meantime, the practical advice remains the same: any pigmented growth that changes rapidly, bleeds without trauma, or looks different from your other SKs should be seen by a dermatologist, not dismissed as another barnacle.
The Cosmetic and Emotional Side
Doctors tend to reassure patients that SKs are “nothing to worry about,” which is medically accurate but can feel dismissive. For people with dozens of visible growths on their face, neck, or chest, the cosmetic impact is real. SKs can create an impression of prematurely aged skin, and individuals with multiple growths often report lower self-esteem related to their appearance.17Symbiosis: Clinical Research in Dermatology. Symbiosis Strategy of Aesthetic Laser Removal of Multiple Seborrheic Keratosis and Effects on Appearance’ Self-Esteem and Well-Being That study also found that comprehensive removal with CO₂ laser improved both self-esteem and well-being scores. SKs can also catch on clothing and jewelry, become irritated, and bleed, which adds a functional nuisance on top of the cosmetic concern.
The frustrating reality is that removal does not prevent new ones from forming. Because the underlying tendency is genetic and age-related, people who are prone to SKs will keep developing them over time. Most dermatologists frame treatment as maintenance rather than cure: you can clear the current crop, but expect new growths to appear in the years that follow. For some patients, periodic sessions to clear accumulated SKs become part of a routine, much like getting teeth cleaned.
SKs on Darker Skin
Seborrheic keratoses in people with darker skin tones deserve separate mention because they carry extra diagnostic challenges. The condition called dermatosis papulosa nigra, those small, dark, slightly raised bumps common on the cheeks and temples of Black individuals, is actually a variant of seborrheic keratosis. These are often smaller and more numerous than the classic SK and can appear as early as the 20s and 30s, considerably younger than the typical onset age. They are equally benign, but their dark pigmentation and early onset sometimes lead to confusion with moles or other pigmented lesions. Treatment options are the same, though extra care is needed with cryotherapy and curettage on darker skin because of the higher risk of post-inflammatory hypopigmentation or hyperpigmentation at the treatment site. Laser and light-based treatments, as well as careful electrodesiccation, are often preferred for this reason.
Why the Name “Barnacles” Persists
The marine barnacle metaphor is sticky, pun acknowledged, for a couple of reasons. Like actual barnacles on a ship’s hull, SKs accumulate over time, seem to appear from nowhere, cluster in groups, and resist casual removal. The nickname also carries an emotional honesty that the clinical term does not. “Seborrheic keratosis” sounds alarming to most people. “Barnacles of aging” at least communicates that the growths are a cosmetic nuisance tied to getting older rather than a disease requiring treatment. Dermatologists sometimes use the term deliberately in conversation with patients for exactly this reason: it normalizes the condition and conveys benignity in a way that a Latin-derived diagnosis cannot. Whether that casual framing helps or hurts depends on the patient. For some, it is reassuring. For others, being told their skin looks like a boat hull is not exactly comforting, and they would rather discuss their removal options than be told not to worry.