Skin discoloration happens when your body produces too much or too little melanin, when blood leaks into surrounding tissue, or when outside substances deposit pigment directly into your skin. A global survey of 48,000 adults across 34 countries found that half of all respondents had at least one pigmentary disorder, making this one of the most common skin concerns worldwide. The causes range from sun exposure and hormones to infections, medications, and nutritional deficiencies.
How Your Skin Makes (and Overproduces) Pigment
Melanin, the pigment responsible for your skin, hair, and eye color, is manufactured inside specialized cells called melanocytes. These cells contain tiny compartments where an enzyme converts the amino acid tyrosine through a chain of chemical reactions into two types of pigment: a dark brown-black form and a lighter yellow-red form. That first enzyme is the rate-limiting step, meaning it controls how fast or slow the entire process runs.
When something ramps up this enzyme’s activity, your skin produces more pigment than usual. UV radiation is the most common trigger. Sunlight damages DNA inside skin cells, which activates a repair protein called p53. That protein flips on a cascade of signals that boost the pigment-producing enzyme, ultimately darkening exposed skin. This is the basic biology behind tanning, sun spots, and many forms of hyperpigmentation.
Sun Damage and Solar Spots
UV radiation is implicated in up to 50% of hyperpigmentation cases. Years of cumulative sun exposure cause melanocytes in certain areas to become overactive, producing flat brown spots commonly called sun spots, age spots, or solar lentigines. In the global survey, solar lentigines were the single most common pigmentary disorder, affecting 27.5% of respondents. These spots tend to appear on the face, hands, chest, and shoulders, wherever your skin has seen the most sunlight over a lifetime.
Hormonal Changes and Melasma
Melasma produces symmetrical patches of darker skin, usually across the cheeks, forehead, upper lip, or chin. It affects roughly 10.9% of adults globally and is strongly tied to hormonal shifts. Estrogen and progesterone both directly increase melanin production by activating the same pigment-producing enzymes that UV light stimulates, but through a separate set of chemical signals.
This is why melasma so frequently appears during pregnancy (affecting 14.5% to 56% of pregnant women depending on the population studied) and in people using oral contraceptives (11.3% to 46%). A study of 324 women found that only 20% of melasma cases actually began during pregnancy, while about 10% first appeared after menopause, showing that hormonal fluctuations at any life stage can be responsible. Melasma can fade on its own once hormone levels stabilize, but it often returns with renewed sun exposure.
Post-Inflammatory Hyperpigmentation
When your skin is injured or inflamed, whether from acne, eczema, a burn, a scrape, or even an aggressive cosmetic procedure, the healing process itself can leave behind dark marks. This is post-inflammatory hyperpigmentation (PIH), and it affected 14.8% of respondents in the global survey.
The mechanism is straightforward: inflammation releases signaling molecules, prostaglandins, and reactive oxygen species that overstimulate nearby melanocytes. Those melanocytes pump out excess pigment, which gets deposited in surrounding skin cells. In some cases, pigment leaks deeper into the dermis, where it can persist for months or years. PIH is more common and more noticeable in darker skin tones because the melanocytes are already more active and respond more vigorously to inflammatory signals.
Vitiligo and Autoimmune Pigment Loss
Not all discoloration involves darkening. Vitiligo causes patches of skin to lose color entirely, creating stark white areas that can appear anywhere on the body. It affected 7.1% of survey respondents worldwide. Vitiligo is an autoimmune condition in which the immune system’s CD8+ T cells, a type of white blood cell normally responsible for fighting infected cells, mistakenly target and destroy melanocytes. Research has shown that these specific T cells are both necessary and sufficient for melanocyte destruction: when isolated from affected skin and placed on unaffected skin in lab settings, they infiltrate the tissue and kill melanocytes on their own.
The triggers that cause this immune misdirection aren’t fully understood, but the process involves an initial stress response within melanocytes that flags them as threats, followed by the immune system mounting a targeted attack.
Fungal Infections
Tinea versicolor, caused by an overgrowth of Malassezia yeast that naturally lives on your skin, creates scattered patches that can be lighter or darker than your surrounding skin. The name “versicolor” literally refers to its ability to change skin color in either direction. In lighter patches, the yeast produces a compound called azelaic acid that inhibits or damages melanocytes, preventing them from responding normally to sun exposure. This is why the light patches become most obvious in summer, when the surrounding skin tans but the affected areas don’t. Darker or reddish patches, on the other hand, result from an inflammatory response to the yeast overgrowth. The patches are typically scaly and appear most often on the chest and back.
Blood Leaking Under the Skin
Some discoloration has nothing to do with melanin at all. When blood pools beneath the skin from an injury or from chronic venous insufficiency (poor circulation in the legs), the body breaks down red blood cells and releases the iron they contain. That iron forms a protein called hemosiderin, which has a rusty, brownish-yellow color. Over time, hemosiderin staining can intensify from yellow-brown to dark brown or even black. Unlike a bruise, which follows the familiar green-to-yellow fading pattern and resolves within weeks, hemosiderin staining can be permanent. It’s particularly common on the lower legs and ankles in people with circulation problems.
Medications That Change Skin Color
A wide range of drugs can cause skin discoloration as a side effect. The list includes antibiotics, antidepressants, anti-seizure medications, antipsychotics, anti-inflammatory drugs, and chemotherapy agents. The mechanisms vary. Some drugs or their breakdown products deposit directly into the skin layers, physically tinting the tissue. Hydroxychloroquine (an antimalarial), amiodarone (a heart medication), and minocycline (an antibiotic) are particularly known for this type of direct deposition, with the drug concentrating deep in the dermis and around small blood vessels.
Other drugs cause discoloration through inflammation. Bleomycin, a chemotherapy agent, produces a distinctive streaky pattern called flagellate erythema. Tetracycline antibiotics can cause darkening specifically in sun-exposed areas. Some medications trigger fixed drug eruptions, where the same spot on the skin darkens each time you take the drug. These reactions can happen weeks to months into treatment and may or may not fade after the medication is stopped.
Vitamin B12 Deficiency
A less well-known cause of darkening skin is vitamin B12 deficiency. When B12 levels drop, melanocytes experience increased oxidative stress and depleted antioxidant defenses. Lab studies have shown that melanocytes under B12-deficient conditions increase melanin production to about 131% of normal levels, with the key pigment-producing enzyme running at 135% of its usual activity. The resulting hyperpigmentation tends to appear in characteristic locations: the backs of the hands and feet, fingers, knees, the sides of the legs, skin folds, and the inside of the mouth. The good news is that this discoloration is typically reversible once B12 levels are restored.
Skin-Lightening Products Gone Wrong
Hydroquinone, the most widely used ingredient in skin-lightening creams, can paradoxically cause a condition called exogenous ochronosis when used for too long. Instead of lightening the skin, the treated area progressively darkens. Microscopic examination reveals yellow-brown pigment deposited in the dermis along with damaged collagen and elastic fibers. The exact chemical process behind this reversal isn’t fully understood, but the clinical recommendation is clear: hydroquinone-containing products should not be used for longer than two years, and if no improvement is visible within six months, use should be stopped.
How Depth Affects What You See
The color of skin discoloration often reveals where the pigment or substance sits. Excess melanin in the outer layer of skin (the epidermis) appears tan, brown, or dark brown and tends to have well-defined edges. Pigment that has dropped into the deeper dermis looks blue-gray or ashy and has blurrier borders. Hemosiderin deposits look rusty brown to black. Dermatologists use a UV light called a Wood’s lamp to help distinguish between these: the light makes epidermal pigment more obvious while deeper pigment fades from view. Certain infections also reveal themselves under this light. Tinea versicolor glows orange, bacterial infections from Corynebacterium appear coral red, and Pseudomonas fluoresces green.
Understanding the depth and type of discoloration matters because it directly affects how responsive the condition will be to treatment. Epidermal pigment is generally more treatable than dermal pigment, which can take months to years to fade or may be permanent.