Dark patches of skin often occur due to hyperpigmentation, which happens when the skin produces more melanin than usual. Melanin is the pigment that gives skin its color.

Dark patches of skin on the knuckles are common among people with darker skin tones. However, anyone can develop dark knuckles. This symptom is usually harmless, but it can sometimes indicate an underlying health condition.

Sometimes, dark knuckles do not have an obvious cause. However, for some people, dark knuckles can be a sign of a metabolic condition, medication side effect, or underlying disease.

In this article, we discuss some of the possible causes of dark knuckles, as well as treatments, home remedies, and when to see a doctor.

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Dark knuckles do not always have an obvious cause.

Acanthosis nigricans

Acanthosis nigricans (AN) is a skin condition that causes small patches of skin, often near the knuckles, to darken, thicken, and become velvety to the touch. AN often takes months or years to appear, and it can cause the skin to have a bad odor or itch.

Other areas where AN develops include:

Some people are more likely than others to get AN. These individuals include people who:

  • have overweight or obesity
  • are of Native American, African, Caribbean, or Hispanic descent
  • have a family history of AN

AN is not contagious or harmful on its own, but it can be a precursor to other medical issues, such as:

Prediabetes or diabetes

Darkening skin can signify that a person is starting to develop diabetes. Many of those most at risk for AN are also at higher risk for diabetes.

Prediabetes means that a person’s blood sugar levels are higher than normal. If a doctor is able to identify prediabetes early, a person may be able to prevent full diabetes from developing by making lifestyle changes.

Hormonal conditions

AN can also signal a thyroid or adrenal condition or a hormonal imbalance, such as polycystic ovary syndrome (PCOS). PCOS causes females to produce too much testosterone, leading to fertility issues, pelvic pain, acne, weight gain, and menstrual irregularities.

Cancer

If AN develops suddenly, it may be a sign of cancer, particularly cancer of the stomach, liver, or colon.

Dermatomyositis

Dermatomyositis is a rare disease that causes muscle weakness through chronic muscle inflammation. One of the first and most recognizable symptoms is a patchy bluish-purple or red rash in areas of skin covering the muscles near the joints. These include the muscles near the knuckles, elbows, heels, and toes.

Other symptoms include:

  • muscle weakness and pain
  • deposits of calcium under the skin, called calcinosis
  • hard, painful nodules
  • unexplained weight loss
  • fever
  • inflamed lungs

Dermatomyositis is not curable, but early detection can help relieve the symptoms and slow down its progression.

Addison’s disease

Addison’s disease is an autoimmune condition in which the body mistakenly attacks the adrenal glands, resulting in low levels of cortisol and aldosterone.

An initial symptom of Addison’s disease is areas of darker skin near scars and skin creases, such as the knuckles. This discoloration provides an early clue, as it can develop months or years before any other symptoms.

Other symptoms of Addison’s disease include:

  • fatigue
  • vomiting and, sometimes, diarrhea
  • nausea
  • abdominal pain
  • mood changes
  • irregular menstrual periods

A person can manage Addison’s disease with hormone replacement therapy to replenish cortisol and aldosterone in the body.

Vitamin B-12 deficiency

Dietary factors can also cause dark knuckles. A 2016 study showed that knuckle pigmentation is an external sign of a vitamin B-12 deficiency. Other symptoms of a vitamin B-12 deficiency include:

  • fatigue
  • weakness
  • constipation
  • loss of appetite
  • weight loss
  • numbness and tingling in the hands and feet

A person with a diagnosed vitamin B-12 deficiency can adjust their diet to include more foods that are high in vitamin B-12, including:

  • fish
  • meat
  • poultry
  • eggs
  • milk

Vegetarians and vegans will likely need to add vitamin B-12 supplements or fortified foods to their diet to meet their daily requirements.

Raynaud’s disease

The main symptom of Raynaud’s disease is blood vessels in the fingers or toes narrowing in response to stress or the cold. This phenomenon can trap blood in the fingers, causing blue discoloration.

Raynaud’s disease sometimes occurs on its own, but it can also be a secondary symptom of another health condition, such as scleroderma or arthritis.

Drug reaction

One case study found that specific drugs are more likely than others to cause hyperpigmentation. These include:

Medication is more likely to cause hyperpigmentation if a person takes more than one type of drug.

Once a doctor identifies the underlying cause of dark knuckles and prescribes treatment, the discoloration should improve. If a medication is causing the hyperpigmentation, a doctor can offer an alternative drug or dosage.

If there is no underlying cause, or a doctor decides not to treat the hyperpigmentation, home remedies can help lighten dark knuckles.

Although dark knuckles themselves do not medically require treatment, people who wish to try at-home treatments to lighten dark knuckles can try the following:

Turmeric

Curcumin is an active compound in turmeric. A 2012 study found that curcumin interrupts the tyrosinase enzyme. This specific enzyme synthesizes melanin, so inhibiting it may help minimize skin darkening.

To use turmeric, a person can combine a small amount of water with this spice and apply the paste to their knuckles for a few minutes.

Tea

A person can also use tea to lighten darker areas of skin. A 2015 study showed that one of the compounds in black, white, and green tea — epigallocatechin gallate — also inhibits tyrosinase and so can help lighten dark knuckles.

A person should steep a tea bag in hot water, let it cool, and then soak a cotton ball in the tea. They can apply the liquid to the knuckles a few times a day.

Vitamin C

Vitamin C is an antioxidant that can help with a variety of skin concerns, including hyperpigmentation. Many skin care products on the market contain vitamin C.

Research suggests that the most effective vitamin C products have a concentration of 10–20%. Higher strengths do not increase the vitamin’s biological significance and may cause irritation.

Alternatively, if a person would rather use an over-the-counter skin product, the American Academy of Dermatology recommend looking for formulas that contain:

  • azelaic acid
  • glycolic acid
  • kojic acid
  • retinoids (retinol, tretinoin, adapalene gel, or tazarotene)

It is not possible to prevent many of the underlying conditions that cause dark knuckles. However, a person can help prevent hyperpigmentation by using sunscreen before going outside.

Avoiding medications that cause hyperpigmentation may also help, but a person should speak to their doctor before stopping a medication.

As darkening knuckles can indicate an underlying health condition, it is important to see a doctor about sudden changes to this area of the skin. If a person has noticed other symptoms, a doctor will be able to find the cause.

Dark knuckles are not usually a cause for concern and do not themselves require treatment. Sometimes, though, they are a sign of an underlying condition. Once a doctor has determined whether a disease is causing the skin to darken, a person can take steps to address it.

Treating the underlying disease should improve the hyperpigmentation. However, if it persists, or there is no underlying condition to treat, a range of home remedies and products can help people manage dark knuckles.