Lines on your nails are usually harmless, especially if they run lengthwise from base to tip. Vertical ridges are the most common type and often appear naturally with age. But horizontal lines, white bands, dark streaks, and tiny blood-colored lines each tell a different story about what’s happening in your body.
Vertical Ridges: Usually Normal
If the lines on your nails run from the cuticle toward the tip, like tiny grooves in the surface, you’re looking at longitudinal ridges. These are extremely common and, in most cases, completely benign. Think of them as the nail equivalent of wrinkles. As you age, the nail matrix (the tissue under your cuticle that produces the nail) becomes less uniform in how it lays down new cells, creating subtle ridges.
That said, vertical ridges aren’t always just aging. Very dry skin or eczema can produce them. Hypothyroidism sometimes causes thicker, more brittle nails with noticeable ridges. And iron deficiency is a well-known culprit. When iron levels drop low enough, nails can develop ridges and eventually curve inward at the edges, forming a spoon-like shape.
Horizontal Lines: A Sign of Disrupted Growth
Horizontal lines are different. Called Beau’s lines, these are actual dents or grooves that run side to side across the nail. They form when something temporarily stops or slows nail growth at the matrix. Once growth resumes, the nail moves forward with a visible depression marking where it paused.
Common triggers include severe illness, high fever, major surgery, chemotherapy, significant nutritional deficiency, or injury to the nail itself. Eczema around the cuticle can also cause them. If you see a horizontal groove on one nail, it’s likely from local trauma. If you see matching grooves across multiple nails at the same position, something systemic disrupted growth in all of them at once.
Here’s a useful trick: fingernails grow at roughly 3.5 millimeters per month. You can estimate when the disruption happened by measuring the distance from the groove to your cuticle. A line sitting about 7 millimeters from the base likely formed around two months ago.
White Lines Across the Nail
White horizontal lines come in a few varieties, and telling them apart matters.
The most common cause is simple trauma. If you banged or jammed a finger, you might see a partial white mark that grows out with the nail over weeks. These lines typically don’t span the full width of the nail.
Mees’ lines are white bands that stretch across the entire nail width. They’re smooth, not indented, and they migrate toward the tip as the nail grows. Pressing on them doesn’t make them disappear. Historically, they’ve been associated with arsenic and heavy metal poisoning, though they can appear after various toxic exposures or severe systemic illness.
Muehrcke’s lines look similar but behave differently. They come in pairs, and they fade when you press down on the nail. Unlike Mees’ lines, they stay in the same spot rather than growing out, because the problem is in the nail bed beneath rather than in the nail itself. They’re linked to low albumin levels in the blood, which can result from liver disease, kidney problems, or malnutrition.
Dark or Pigmented Streaks
A dark brown or black line running lengthwise along the nail deserves attention. This is called longitudinal melanonychia, and it happens when pigment-producing cells in the nail matrix become active along a narrow band. It’s common and usually harmless in people with darker skin tones, where multiple nails may show faint brown streaks.
Rarely, a dark streak can signal melanoma developing under the nail. Dermatologists use a set of warning signs to evaluate these streaks: the person is between 50 and 70 years old, the band is 3 millimeters or wider with blurry borders, the streak has changed over time, it appears on the thumb, index finger, or big toe, and most importantly, pigment has spread beyond the nail onto the surrounding skin (a finding called the Hutchinson sign). A personal or family history of melanoma also raises concern. A single new dark streak on one nail that’s widening or changing warrants a dermatology visit.
Tiny Blood-Colored Lines
Small, thin, dark reddish-brown lines that look like splinters embedded under the nail are called splinter hemorrhages. They run vertically and usually appear near the fingertip end of the nail. The overwhelming majority are caused by minor trauma you may not even remember, like catching your nail on something.
These lines became famous in medicine because they were first described in patients with endocarditis, an infection of the heart valves. But that association turned out to be far less reliable than initially thought. Splinter hemorrhages show up in fewer than 15% of endocarditis cases, and they occur in plenty of other situations, from everyday bumps to various infections. A single splinter hemorrhage on one nail is almost certainly from trauma. Multiple unexplained splinter hemorrhages across several nails, especially with fever or other symptoms, are worth mentioning to your doctor.
Nutritional Factors and Nail Health
Several nutrient deficiencies show up in your nails. Iron deficiency causes ridges and brittleness. Zinc deficiency can produce white spots and horizontal lines. B12 deficiency sometimes leads to nail discoloration or fragility.
Biotin (vitamin B7) has the most direct evidence for improving nail quality. In clinical studies, 2.5 milligrams of biotin daily for six months or longer increased nail thickness by 25% in people with brittle nails. In one trial of 45 people with thin, brittle nails, 91% reported firmer, harder nails after an average of 5.5 months of supplementation. Results aren’t immediate because you’re waiting for an entirely new nail to grow in. A fingernail takes roughly six months to fully replace itself, and toenails take up to 18 months.
How to Read Your Nails
The direction, color, and number of lines narrow down the cause quickly:
- Vertical ridges on multiple nails: likely age-related or from dry skin, occasionally thyroid or iron issues
- Horizontal dents on multiple nails: a past illness, surgery, or nutritional disruption that temporarily paused growth
- Horizontal dent on one nail: probably local injury to that finger
- White bands spanning the full nail: possible systemic cause worth investigating, especially if on multiple nails
- Dark lengthwise streak on one nail: usually benign pigmentation, but worth monitoring for changes in width or color
- Small reddish-brown vertical lines near the tip: almost always minor trauma
If lines appeared on just one nail after you injured it, they’ll grow out on their own within a few months. If you’re seeing changes across many nails, that pattern points toward something happening internally, whether it’s a nutritional gap, a thyroid issue, or the aftermath of a recent illness.