Skin pigmentation is determined by melanin, a natural pigment produced by specialized cells called melanocytes in the outer layer of your skin. Changes in pigmentation, whether darkening, lightening, or uneven patches, result from shifts in how much melanin your melanocytes produce and how that melanin gets distributed. The triggers range from sun exposure and hormones to genetics, inflammation, medications, and underlying health conditions.
How Your Skin Makes Pigment
Melanin production starts with an amino acid called tyrosine. An enzyme called tyrosinase, the rate-limiting step in the whole process, converts tyrosine into a series of intermediate compounds that ultimately become melanin. This happens inside tiny structures called melanosomes, which sit within your melanocytes. Once melanin is produced, those melanosomes travel outward and transfer their pigment to the surrounding skin cells (keratinocytes), where the melanin forms a protective cap over each cell’s DNA.
There are two main types of melanin. Eumelanin is brown-black and provides the strongest UV protection. Pheomelanin is yellow-red and offers much less. The ratio between these two types largely determines your baseline skin tone, hair color, and how your skin responds to the sun.
Genetics Set Your Baseline
A gene called MC1R plays a central role in determining which type of melanin your melanocytes produce. When the receptor it codes for is activated, it triggers a chain of reactions that stimulate eumelanin production. When the receptor is inactive or blocked, melanocytes default to making pheomelanin instead. Common variants in MC1R reduce eumelanin production, which is why people who carry them tend to have lighter skin, red hair, and freckles. Dozens of other genes also influence pigmentation, but MC1R is one of the most studied and impactful.
UV Exposure and Tanning
Sunlight is the single most powerful environmental trigger for pigmentation changes. When UV radiation hits your skin, it damages the DNA in keratinocytes. Those damaged cells respond by releasing a signaling hormone that binds to receptors on nearby melanocytes, essentially telling them to ramp up melanin production. The melanocytes then synthesize melanin inside their melanosomes and shuttle it outward to surrounding skin cells, where it absorbs future UV radiation and shields the DNA underneath.
This is the biological basis of tanning. It’s not a sign of healthy skin; it’s a damage response. Over years of repeated exposure, melanocytes in certain areas can become permanently overactive or unevenly distributed, leading to sun spots (solar lentigines) and patchy discoloration.
Hormonal Changes and Melasma
Hormones are a major driver of pigmentation, particularly estrogen and progesterone. Melanocytes carry receptors for both of these hormones, and in people prone to melasma, those receptors are more abundant and more sensitive to stimulation.
Estrogen directly increases the production of tyrosinase and related enzymes, accelerating melanin output. It works through two pathways: a slower one that ramps up gene expression for pigment-producing enzymes, and a faster one that activates signaling molecules at the cell membrane to boost melanocyte activity almost immediately. Progesterone contributes through a different mechanism, activating a signaling chain that ultimately increases the expression of the same key enzymes. It also triggers oxidative stress and inflammatory signals that further stimulate melanocytes.
This is why melasma commonly appears during pregnancy (sometimes called the “mask of pregnancy”), while taking oral contraceptives, or during other periods of hormonal fluctuation. The patches typically show up on the cheeks, forehead, upper lip, and chin.
Post-Inflammatory Hyperpigmentation
Any time your skin is inflamed or injured, it can darken in that area afterward. This is called post-inflammatory hyperpigmentation (PIH), and it’s one of the most common causes of uneven skin tone. Acne, eczema, burns, cuts, and even aggressive cosmetic procedures can all trigger it.
During inflammation, your skin releases a flood of signaling molecules, including prostaglandins, interleukins, tumor necrosis factor, and reactive oxygen species like nitric oxide. These molecules stimulate melanocytes in the affected area to produce excess melanin. The darkened patches left behind can last months or even years, particularly in people with deeper skin tones, who are more susceptible to PIH because their melanocytes are already more active at baseline.
Medical Conditions That Affect Pigmentation
Certain systemic diseases cause widespread pigmentation changes. Addison’s disease is one of the clearest examples. In this condition, the adrenal glands can’t produce enough cortisol. The body compensates by increasing production of a precursor hormone that gets cleaved into both ACTH (which tries to stimulate the adrenals) and melanocyte-stimulating hormone. The excess melanocyte-stimulating hormone drives melanin production throughout the body, causing generalized skin darkening that’s often most noticeable in skin creases, scars, gums, and areas exposed to friction.
Thyroid disorders, Cushing’s syndrome, and certain liver conditions can also alter pigmentation, though through different mechanisms.
Vitamin B12 Deficiency
Up to 1 in 5 people with low vitamin B12 levels develop noticeable skin darkening. The hyperpigmentation is most common on the hands and feet, particularly over the knuckles, and can also appear on the palms, soles, and between finger joints. The mechanism involves increased tyrosinase enzyme activity combined with problems transferring melanin from melanocytes to surrounding skin cells, leading to uneven pigment accumulation. The good news: this type of hyperpigmentation is typically reversible once B12 levels are corrected.
Medications That Darken Skin
A surprisingly long list of medications can cause skin pigmentation as a side effect. The discoloration can be patchy or widespread, and it sometimes appears blue-gray, brown, or even yellowish depending on the drug involved. Common culprits include:
- Antimalarials like hydroxychloroquine, often used for lupus and rheumatoid arthritis
- Certain antibiotics, particularly minocycline (a tetracycline-class drug used for acne)
- Heart medications like amiodarone
- Chemotherapy drugs including bleomycin, cisplatin, and doxorubicin
- Anti-seizure medications like phenytoin
- Oral contraceptives, which overlap with the hormonal pathway described above
- Some antidepressants, including tricyclics
- NSAIDs and even paracetamol in some cases
Drug-induced pigmentation sometimes fades after stopping the medication, but not always. The timeline depends on which drug caused it and how long you were taking it.
Aging and Dark Spots
As skin ages, pigmentation becomes more uneven for two distinct reasons. The first involves melanin: decades of cumulative sun exposure cause certain melanocytes to become chronically overactive, producing the flat brown spots commonly called age spots or liver spots. These are technically solar lentigines and are most common on the face, hands, shoulders, and forearms.
The second involves a different pigment entirely. Lipofuscin, sometimes called the “wear and tear” pigment, accumulates in skin cells over time as a byproduct of cellular aging. It creates brown spots that look similar to melanin-based discoloration but are actually filled with lipid deposits. These tend to appear on the backs of the hands, cheeks, and chest. The two types of age-related spots often coexist but arise from completely different processes.
Visible Light and Protection Beyond SPF
Most people think of UV protection when they think of preventing pigmentation, but visible light (the light you can actually see) also contributes to skin darkening. Visible light makes up about 45% of solar radiation, and standard sunscreens, even mineral formulas with high SPF ratings, offer limited protection against it.
This matters most for people with medium to deep skin tones (Fitzpatrick skin types III and above), who are more reactive to visible light. Studies have found that sunscreens containing iron oxide pigments significantly outperform non-tinted mineral sunscreens at blocking visible-light-induced pigmentation. Tinted sunscreens serve a dual purpose: they mask existing discoloration while preventing new pigmentation from developing across both the UV and visible light spectrum.