Smegma is a whitish or yellowish substance that accumulates under the foreskin of the penis or around the clitoral hood. It is made up of shed skin cells, natural oils, and moisture, and it forms as a normal byproduct of genital skin turnover. Despite a persistent reputation as something dirty or dangerous, smegma itself is not inherently harmful, and several medical claims historically attached to it have been debunked. The substance does, however, warrant regular gentle cleaning, because buildup can become a breeding ground for bacteria and a source of irritation.
What Smegma Is Made Of
At its core, smegma consists of dead epithelial cells that the body continuously sheds from the inner surface of the foreskin (or clitoral hood) and the glans. These cells mix with skin oils and trace amounts of moisture to form a soft, paste-like material. In infants and young children, the substance tends to appear as small, firm, whitish lumps sometimes called “smegma pearls,” which form when desquamated cells collect under a foreskin that has not yet fully separated from the glans.1PubMed Central. Smegma pearl: A benign penile lesion in infants
Chemical analyses have shown that the lipid fraction of smegma is dominated by sterols and a hydrocarbon called squalene, which also shows up abundantly in general human skin oil.2Oncology. Lipids of Human and Equine Smegma In adults over 35, smegma may also contain a cyclopropane fatty acid (9,10-methyleneoctadecanoic acid) that is not detectable in younger men’s samples, suggesting the composition shifts somewhat with age.3Oncology. Lipids of Human and Equine Smegma The practical takeaway is that smegma is not some exotic secretion; it is mostly the same dead-skin-and-oil mixture that collects in other warm, folded areas of the body, like behind the ears or between the toes.
Where It Actually Comes From
For centuries, textbooks attributed smegma production to structures called “Tyson’s glands,” described in the 17th century as modified oil glands sitting in the groove behind the head of the penis. The problem is that when researchers actually looked for them, they could not find them. A histological study examining partial penectomy specimens found neither sebaceous glands nor hair-follicle-associated glands on the mucosal surface of the foreskin or at the coronal sulcus.4PubMed. Ectopic folliculosebaceous units at the coronal sulcus A separate prospective study confirmed the same conclusion: smegma was originally thought to come from these glands, but the glands were not found.5PubMed Central. Microbiology of smegma: Prospective comparative control study
What appears to happen instead is simpler than the old gland theory. The inner lining of the foreskin and the surface of the glans are covered in mucosal-type epithelium that sheds cells at a relatively high rate. Those cells, combined with whatever small amount of sebum migrates from nearby keratinized skin, accumulate in the warm, enclosed space under the foreskin. The original 17th-century description of Tyson’s glands was never accompanied by illustrations, and subsequent authors seem to have passed the claim along uncritically for hundreds of years.6PubMed. Ectopic folliculosebaceous units at the coronal sulcus In very rare cases, ectopic sebaceous structures can appear at the coronal sulcus, but these are considered oddities rather than normal anatomy.
The Bacteria That Live in It
Because smegma sits in a warm, moist environment rich in organic material, it unsurprisingly supports bacterial growth. A study of uncircumcised boys in Nigeria cultured smegma samples and found bacteria in roughly 80% of them. About two-thirds of positive samples contained a single bacterial species, while around 15% harbored a mix. The most common gram-negative organism was E. coli, accounting for about 90% of gram-negative isolates, while Staphylococcus epidermidis and Staphylococcus aureus together made up most of the gram-positive findings.7PubMed. Microbiology of smegma in boys in Kano, Nigeria
The bacterial profile matters because chronic irritation from smegma buildup is recognized as one factor that can contribute to balanitis and balanoposthitis, conditions where the glans and foreskin become inflamed. Beyond bacteria, the subpreputial environment also hosts yeasts. Research on children found that uncircumcised boys had significantly higher rates of colonization by lipid-loving Malassezia yeasts compared to circumcised boys, with circumcision status changing the colonization rate by nearly fivefold.8Medical Mycology. Effect of circumcision on genital colonization of Malassezia spp. in a pediatric population These yeasts feed on the lipids present in smegma, which likely explains the association. None of this means that having smegma automatically leads to infection, but allowing large amounts to accumulate does shift the microbial balance in a direction that raises the odds of irritation.
Smegma and Cancer: A Myth That Persisted for Decades
One of the most damaging claims about smegma is that it causes penile cancer. This idea took root in the mid-20th century and was repeated so often that it became treated as established fact in some medical literature. When the original animal and human studies behind the claim were reanalyzed using proper statistical methods, the evidence fell apart. A dedicated review concluded that assertions of smegma’s carcinogenicity “cannot be justified on scientific grounds.”9PubMed. The carcinogenicity of smegma: debunking a myth
Penile cancer does have real, well-documented risk factors, but smegma on its own is not one of them. The established risks include phimosis (a foreskin too tight to retract), HPV infection, chronic inflammation, smoking, lichen sclerosis, and compromised immune function.10PubMed Central. Updates on the epidemiology and risk factors for penile cancer Prevention strategies focus on addressing these factors directly: treating phimosis, HPV vaccination, condom use, smoking cessation, and basic hygiene.11PubMed. Penile cancer: epidemiology, pathogenesis and prevention
How did the myth survive so long? Part of the answer is that smegma buildup and phimosis tend to occur together, and phimosis genuinely is a risk factor for penile cancer. The chronic inflammation that results from a tight, non-retractable foreskin trapping smegma and bacteria is the actual concern, not the smegma itself. Blaming the substance rather than the underlying condition was an easy shorthand that proved remarkably hard to dislodge from textbooks.
Smegma Pearls in Infants
Parents sometimes notice small, firm, yellowish-white lumps under the foreskin of infant boys and panic. These “smegma pearls” are benign collections of shed epithelial cells that form naturally as the foreskin gradually separates from the glans during early childhood.12PubMed Central. Smegma pearl: A benign penile lesion in infants In newborns, the inner foreskin and glans are fused together, and the separation process can take months or years. As it occurs, pockets of dead cells collect and sometimes become visible through the skin.
These pearls require no treatment and no attempt to retract the foreskin to remove them. Forcible retraction of an infant’s foreskin can cause tearing, scarring, and adhesions that create real problems where none existed. The pearls work their way out on their own as the foreskin continues to loosen naturally. Pediatricians familiar with foreskin development will recognize them immediately, but parents who have never seen them before sometimes mistake them for cysts or signs of infection.
Female Smegma
Smegma is not exclusive to penile anatomy. The same process of epithelial shedding and oil accumulation happens around the clitoris, under the clitoral hood. In most people, routine washing keeps it from building up, but in cases of clitoral phimosis (where the hood is abnormally tight or adhered), smegma can accumulate in substantial quantities. Case reports describe patients with severe clitoral phimosis developing large smegmatic pseudocysts, where a pocket of trapped smegma forms a visible mass that can be mistaken for something more serious. In one documented case, copious smegma was irrigated from a cyst, and no adhesions were found underneath.13The Journal of Sexual Medicine. Patient with Severe Clitoral Phimosis and Smegmatic Cyst: A Case Report and Review of the Literature
Female smegma tends to receive less medical attention than its male counterpart, partly because the anatomy is less visible and partly because the foreskin-related hygiene discussion in medicine has historically focused on circumcision debates. But the practical guidance is the same: gentle, regular cleaning of the area with warm water is sufficient to prevent buildup. Harsh soaps and antiseptics can disrupt the local microbial balance and cause more irritation than the smegma itself would.
How to Clean It (and How Not To)
Smegma washes away easily with warm water and, if desired, a mild unscented soap. For uncircumcised men and boys old enough to retract their foreskin, gently pulling it back, rinsing, and replacing it is all that is needed. For women, the same principle applies to the clitoral hood. The emphasis on “gentle” and “mild” is not casual: the mucosal skin in these areas is thinner and more sensitive than regular external skin, and aggressive scrubbing or strong cleaning products can strip away protective moisture and provoke irritation or allergic reactions.
A few specifics worth knowing:
- Frequency: Daily washing during bathing is enough for most people. There is no benefit to cleaning more aggressively.
- Infants: Do not forcibly retract the foreskin to clean underneath. Clean only what is visible and easily accessible.
- Products: Plain warm water works well on its own. If using soap, choose one without fragrance or dyes. Antiseptic washes and alcohol-based products are overkill for routine care and may cause contact dermatitis.
- After cleaning: Gently dry the area and return the foreskin to its resting position. Leaving it retracted can cause paraphimosis, where the foreskin gets stuck behind the glans and swells.
When smegma has been allowed to accumulate and harden, it can become more difficult to remove. Soaking in warm water before gently cleaning usually softens it enough to come away without force. If hardened smegma is stuck to the skin and resists gentle cleaning, a healthcare provider can help rather than risk tearing delicate tissue.
Smegma in Other Animals
Smegma is common across mammals, particularly in species with a prepuce or sheath covering the penis. Horse owners are probably the most familiar with this reality: stallions and geldings accumulate smegma inside the penile sheath, and the question of how often (and whether) to clean it is a regular topic in equine veterinary care. Chemical analysis of horse smegma found a similar lipid profile to the human version, with squalene as the main hydrocarbon, though the horse material contained a higher proportion of sterols and a wider range of branched-chain alcohols.14Oncology. Lipids of Human and Equine Smegma
Dogs also accumulate preputial smegma, and a greenish-yellow discharge from the sheath is normal in intact males. Veterinary research has examined the cellular composition of canine preputial smegma and found that the proportion of cornified cells varies with factors like age and cleaning.15PubMed Central. Factors affecting the differential epithelial cell count in preputial cytology of healthy intact dogs In both horses and dogs, the veterinary consensus leans toward minimal intervention: occasional cleaning when buildup is excessive, but not so frequent as to disrupt the local microbial environment. The parallel to human care is striking. Across species, the basic message is the same: some smegma is normal, too much causes trouble, and overcleaning creates its own problems.
The Pheromone Connection
There is a less-discussed angle on smegma that touches on chemical signaling. Researchers have identified that substances similar to androstenone, a steroid compound well established as a pheromone in other mammals, are secreted in human smegma as well as in the apocrine glands of the underarm and pubic regions.16ScienceDirect. Pheromones in sex and reproduction: Do they have a role in humans? Related male pheromone compounds have been found concentrated in human semen, and some research suggests women are most sensitive to these compounds during their fertile window.
Whether human smegma plays any functional role in sexual attraction is far from settled. The entire field of human pheromone research is contested, with strong skeptics arguing that humans lack the specialized vomeronasal organ that other mammals use to detect pheromones. What is clear is that the subpreputial environment produces compounds that in other species serve a signaling function. Whether evolution has preserved any version of that function in humans, or whether it is a vestigial chemical remnant with no practical effect, remains an open question. It is an intriguing footnote to the biology of a substance most people would rather not think about at all.
When to See a Doctor
Smegma on its own is not a medical problem, but certain situations warrant professional attention. Redness, swelling, pain, or discharge with an unusual color or odor may signal balanitis or another infection that needs treatment. Persistent inability to retract the foreskin in an adult (phimosis) traps smegma and creates a cycle of buildup, bacterial overgrowth, and inflammation that often requires medical intervention to break. In women, clitoral adhesions or phimosis that prevent normal cleaning should be evaluated, especially if accompanied by pain or a palpable mass.
Children present a slightly different picture. Because foreskin separation happens gradually and on its own timeline, smegma pearls and incomplete retractability in young boys are almost always normal. The red flag in children is not the presence of smegma but signs of active infection: pain with urination, visible swelling, redness spreading beyond the foreskin, or fever. If a child is comfortable and the foreskin appears healthy, the smegma is doing exactly what it is supposed to do during the natural separation process.