Viral Reddit nail photo sparks cancer fears: Are faint black lines on nails dangerous or usually harmless?

A simple photo shared on Reddit has turned into a major health discussion online, after users noticed faint black lines running through two nails.

The woman posted the image in r/mildlyinteresting with a short caption pointing out the dark streaks on her thumb and a toenail. She did not share it in a medical forum, but many commenters treated it like a warning sign anyway, urging her to see a doctor quickly.

Why a “black line” on a nail gets so much attention

Dark streaks on nails can be harmless, but they can also appear in rare cases of nail melanoma, also called subungual melanoma, which develops under the nail. Because melanoma has a reputation for spreading quickly if missed, social media users often jump straight to worst-case assumptions when they see unexplained pigmentation. 

That fear is understandable, but it can also create unnecessary panic. Even medical professionals note that online comments are not a diagnosis.

The most common explanation: melanonychia

A dark line or band on a nail is often referred to as melanonychia, a brown-to-black discoloration caused by melanin deposits in the nail plate. It can affect one nail or several nails. 

Experts list several non-cancerous causes, including:

• A bruised nail or past injury
• Repeated minor trauma (for example, friction from tight shoes)
• A mole in the nail area
• “Age spot” type changes in the nail
• Pregnancy
• Certain medications (including some chemotherapy and other drug types)
• Nutritional deficiencies such as vitamin B12 deficiency
• Fungal or bacterial infections and inflammatory skin conditions 

Doctors also note melanonychia is more common in people with medium to dark skin tones, and in some cases can be linked to genetics and natural pigmentation patterns rather than disease. 

When doctors worry about nail melanoma

While most nail streaks are benign, clinicians pay close attention when a band looks irregular or changes over time.

Cleveland Clinic notes nail melanoma often starts as a small dark band under the nail that can gradually widen or become uneven. It may also be associated with nail damage such as splitting or cracking. In more advanced cases, bleeding can occur, and there may be pigment spreading to the skin around the nail (often called Hutchinson sign). 

Other warning signs that can raise concern include:

• Swelling or inflammation near the nail
• The nail lifting from the nail bed
• Ulcers, nodules, or bleeding
• A noticeable change in the band’s color, width, or shape over time 

Importantly, Cleveland Clinic notes the cause of subungual melanoma is not fully understood and, unlike many other melanomas, it does not appear to be driven primarily by sun exposure. Injury or trauma may be a risk factor in some cases. 

One nail or several nails: what it can suggest

People online often assume “one nail means cancer,” but real-life assessment is more nuanced.

Melanonychia can appear in one or multiple nails, and when multiple nails are involved, it may point toward benign pigmentation patterns, especially in people with darker skin tones. However, any new streak or changing streak still deserves evaluation. 

In the Reddit post, the streaks appeared in two different nails. Some commenters suggested that two separate nail melanomas at the same time would be unlikely, while still agreeing that a professional exam and possible biopsy would be the sensible next step.

What getting it “checked” usually means

For suspicious nail streaks, clinicians may:

• Review your history (injury, new medications, changes over time)
• Examine the nail closely and compare to other nails
• Use dermoscopy (a magnified exam tool)
• Recommend a biopsy when the appearance warrants it, because a biopsy is the definitive way to rule out melanoma 

What about UV nail lamps and gel manicures?

The viral discussion also revived a common question: can gel manicure lamps increase cancer risk?

Multiple experts note nail lamps emit UVA radiation, but at much lower levels than tanning beds. Current evidence suggests the cancer risk from occasional use appears low, though repeated lifetime exposure can add up, and people at higher skin-cancer risk may want to reduce UV exposure where possible. 

The bigger takeaway: don’t panic, but don’t ignore changes

A faint line on a nail is not automatically a cancer diagnosis. Still, doctors emphasize that unexplained nail changes should not be brushed off, especially if the streak is new, widening, irregular, or paired with nail damage or bleeding.

The woman who posted the image has not publicly shared an update about whether she saw a doctor, but her post has already served as a reminder of a useful rule: social media can raise awareness, but only a healthcare professional can evaluate the cause.

Medical note: This article is for general information only and is not a substitute for professional medical advice. If you notice a new or changing dark line on a nail, consider booking an evaluation with a qualified clinician.