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How To Tell If a Mole is Cancerous: 5 Signs You Need to Know

Written by 
Lacey Ramburger

Article at a glance

  • On average, a person develops 10–40 moles in their lifetime.

  • Moles are usually harmless, though some have the potential to become cancerous.

  • An unusual mole can potentially indicate melanoma skin cancer.

  • Early detection can play a vital role in detecting/​treating melanoma.

What Are Moles?

Moles are pigmented clumps of cells that show up on various parts of the body. Throughout a person’s life, their body may produce between 10 and 40 moles. Moles are relatively common and are not an immediate cause for concern.

Moles come in different shapes, sizes, and colors and can appear almost anywhere on the body. Moles are usually harmless, but in some cases, they can point to signs of skin cancer. So how does a person tell the difference between normal moles and cancerous ones?

What Is Skin Cancer?

Skin cancer is excessive, abnormal growth of cells in the outermost layer of skin, also called the epidermis. DNA damage that hasn’t repaired itself causes skin cells to rapidly multiply and form malignant tumors. One of the early signs of skin cancer is the presence of unusual moles.

Types of Skin Cancer

There are a few types of skin cancers, including melanoma, squamous cell carcinoma, basal cell carcinoma, and Merkel cell carcinoma. 


Melanoma skin cancer develops from melanocytes, or skin cells that produce the skin pigment melanin. It is considered the most dangerous form of skin cancer.

Squamous Cell Carcinoma

This skin cancer develops from the epidermis’s uncontrollable growth of squamous cells.

Basal Cell Carcinoma 

This skin cancer develops from the uncontrollable growth of basal cells in the epidermis.

Merkel Cell Carcinoma 

This skin cancer is associated with the virus Merkel cell polyomavirus and is considered a rare form of skin cancer.

Note: Out of the four types of skin cancer, melanoma is the one that potentially develops from existing moles (as well as areas of previously normal skin).

Melanoma Signs

While melanoma is the third most common skin cancer, it is also the most deadly—meaning it’s vital to know the potential signs to treat the cancer as quickly as possible. Some of the potential melanoma warning signs include:

  • New, unusual mole or mark on the skin

  • Changes in existing or surrounding moles, particularly in size, shape, or color

  • A mole or mark that looks drastically different from other spots on your skin (also referred to as the ugly duckling” rule)

  • A sore that doesn’t heal

It’s important to know that melanoma doesn’t always present in these forms, so it’s crucial to have new or unusual marks on your body checked regularly.

Determining if a Mole is Cancerous: 5 Warning Signs

In many cases, moles are ultimately harmless. However, some melanomas develop from existing moles (though this is rare), so if a mole suddenly seems to change, this could be a cause for concern.

So how can a person determine if their mole is cancerous? One of the most common ways is to use the ABCDE method:

A: Asymmetry — Most normal moles usually maintain an even shape and appearance on both sides. If one half of a mole looks different from the other, this could indicate the appearance of an unusual mole.

B: Border — Typically, the average mole has a well-defined border, while cancerous moles potentially have borders that are fuzzy or uneven around the edges,

C: Color — Moles can come in several different shades, including brown, black, tan, or even pink. If a mole seems to change color drastically, this could indicate a need to have the mole inspected.

D: Diameter — the average mole is usually under 6 millimeters, approximately the size of a pencil eraser. If a mole is larger than this, it should be checked out.

E: Evolution — if an existing mole is changing rapidly—in color, size, shape, or presentation—it could be a sign of a cancerous mole and should be checked out right away.

Risk Factors and Causes of Melanoma

The main cause of melanoma is excessive amounts of Ultraviolet (UV) light on the skin. This can happen due to direct sun exposure or due to tanning beds.

However, some risk factors could potentially cause a person to develop melanoma, including 

  • Fair skin

  • Family history of melanoma

  • Previous personal history of melanoma

  • Large numbers of freckles and/​or moles

  • Those over the age of 40

  • Those with compromised/​weakened immune systems

Note: while those with lighter skin are at higher risk of developing melanoma, those with darker skin are at higher risk for certain subtypes, including acral lentiginous melanoma.

How is Melanoma Diagnosed?

If you suspect that you have an unusual mole, your doctor will take a closer look at the concerned area. If they believe there is potential for skin cancer, they will likely refer you to a dermatologist to run some tests, including those for melanoma.

Additionally, a biopsy may be done that involves removing a piece of the mole to have it tested. The results should determine whether you have melanoma.

When to See a Doctor

It’s important to see your doctor if any of the following occurs

  • An unusual mole or mark appears unexpectedly, especially if it looks different than other moles on the skin

  • If an existing mole changes size, shape, color, or texture, especially at a rapid rate

  • If an existing mole begins to itch, become painful, or start bleeding

Reduce Melanoma Risk

There are a few things you can do to potentially reduce your risk of melanoma, including

  • Wear sunscreen: if you plan to be outdoors, apply sunscreen every few hours to ensure you don’t develop a sunburn.

  • Use tanning beds less often: tanning beds are responsible for high amounts of UV radiation, so eliminating or reducing your time in the tanning beds can help reduce skin exposure.

  • Make regular appointments with a dermatologist: have a dermatologist regularly examine your skin for any unusual marks or moles.

  • Have unusual marks checked immediately: the longer you wait to have a mole examined, the more severe melanoma can become. The earlier melanoma is detected, the less fatal it will be. 


Are all moles cancerous?

No, not all moles are cancerous. In fact, most moles aren’t a cause for concern—though if a mole looks unusual, it should be inspected as soon as possible.

Can a doctor tell if a mole is cancerous just by looking at it?

If a mole looks extremely unusual—such as abnormal size, color, or shape—a doctor may be able to have strong confidence that a person has a cancerous mole. However, tests and a biopsy will confirm any suspicions.

Is melanoma deadly?

Melanoma is considered the most deadly type of skin cancer, as it can affect other areas of the body beyond the skin, such as the lungs, brain, or liver. Early detection can help reduce the fatal risk of melanoma.

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