Red lines running along your nails are most often splinter hemorrhages, tiny streaks of blood trapped beneath the nail plate. They get their name because they look like thin splinters embedded under the nail. In most cases, they result from minor trauma you may not even remember, and they grow out harmlessly as your nail grows. Less commonly, they signal an underlying health condition worth investigating.
What Splinter Hemorrhages Look Like
These lines are thin, reddish-brown streaks that run vertically along the length of your nail, following the direction of nail growth. They’re caused by bleeding from tiny blood vessels in the nail bed. Fresh ones tend to look red, while older ones darken to a brownish or blackish color as the blood dries.
Where the line sits on your nail matters. Lines near the tip of your nail are almost always from everyday trauma: bumping your hand, biting your nails, or using your fingers as tools. Lines that appear closer to the base of the nail, near the cuticle, are more likely tied to a systemic condition, especially if they show up on multiple fingers at the same time.
Trauma Is the Most Common Cause
The vast majority of splinter hemorrhages come from physical injury to the nail. You don’t need a dramatic event to cause one. Repeated tapping on a keyboard, gripping tools, wearing tight shoes, or even vigorous hand-washing can produce enough force to rupture the delicate capillaries under the nail. Many people notice one and can’t recall any injury at all.
Trauma-related splinter hemorrhages resolve on their own as the nail grows forward. In a study tracking toenail splinter hemorrhages over six months, more than half disappeared within two months, and about 95% were gone within 24 weeks. Fingernails grow faster than toenails, so fingernail lines typically clear sooner. The longest recorded case in that study took 11 months to fully grow out.
Medical Conditions That Cause Red Lines
When red lines aren’t from trauma, they can point to conditions affecting your blood vessels, immune system, or skin. The key clues are lines appearing on several nails at once, recurring after they grow out, or showing up near the base of the nail rather than the tip.
Skin Conditions
Nail psoriasis is one of the more common culprits. It causes splinter hemorrhages along with other nail changes like pitting (small dents), thickening of the skin under the nail, and an “oil drop” discoloration where part of the nail turns yellowish-brown. If you already have psoriasis patches on your skin or scalp, nail involvement is especially likely. Lichen planus, another inflammatory skin condition, can also produce red lines on nails.
Blood Vessel Inflammation
Conditions that inflame blood vessels, grouped under the term vasculitis, can trigger splinter hemorrhages because the tiny capillaries in the nail bed become fragile and leak. Lupus and antiphospholipid syndrome are two autoimmune diseases known to cause this. Chronic kidney disease is another association.
Heart Valve Infection
Infective endocarditis, a bacterial infection of the heart valves, is the most serious condition linked to splinter hemorrhages. In this case, tiny clots break off from the infected valve and travel through the bloodstream, lodging in the small vessels under the nails. These hemorrhages tend to appear on multiple nails and sit closer to the cuticle. Other symptoms like fever, fatigue, and unexplained weight loss would typically be present too.
Medications
Certain drugs make splinter hemorrhages significantly more likely. Blood thinners like warfarin and aspirin increase the chance of bleeding under the nail from minor impacts. A class of cancer drugs called kinase inhibitors causes splinter hemorrhages in 60% to 70% of people taking them.
Red Lines That Run the Full Length of the Nail
If the red line extends from the cuticle all the way to the tip of your nail as a continuous, narrow band, this is a different finding called longitudinal erythronychia. Unlike splinter hemorrhages, which are short streaks of trapped blood, this is a persistent red stripe caused by changes in the nail bed tissue itself.
A single red stripe on one nail can sometimes indicate a small growth underneath called a glomus tumor. These are benign, but they’re notable for the intense pain they cause. The classic pattern is a throbbing ache under the nail, sharp sensitivity to cold temperatures, and severe pain from even gentle pressure or minor bumps. If you have a painful red or blue-red line under one nail, this is worth mentioning to a doctor because surgical removal resolves the pain completely.
When the same type of red stripe appears on multiple nails, it’s more likely connected to a broader condition like lichen planus or Darier disease, a genetic skin disorder.
How to Tell Harmless Lines From Concerning Ones
Most red lines under your nails are nothing to worry about. A few features can help you sort the harmless from the worth-investigating:
- Single nail vs. multiple nails. A line on one nail, especially after you remember bumping it, is almost certainly trauma. Lines appearing on several nails at once deserve a closer look.
- Location on the nail. Lines near the free edge (tip) lean toward trauma. Lines near the cuticle lean toward systemic causes.
- Other nail changes. If the red lines come with pitting, thickening, lifting, or discoloration of the nail, a skin condition like psoriasis may be at play.
- Pain. Splinter hemorrhages from trauma are painless. A painful red or blue-red line under a single nail raises the possibility of a glomus tumor.
- Growth over time. Trauma-related lines move forward as your nail grows and eventually disappear at the tip. Lines that stay in the same position or get wider aren’t behaving like trapped blood and should be evaluated.
Dark Lines vs. Red Lines
It’s worth noting the difference between red lines and dark brown or black lines. A dark streak running from the base of the nail to the tip, especially one that’s widening over time or causing pigment to spread onto the surrounding skin (called the Hutchinson sign), can be a sign of subungual melanoma, a type of skin cancer that develops under the nail. This looks distinct from a splinter hemorrhage: melanoma streaks are typically darker, have irregular pigmentation, and grow slowly over weeks to months rather than appearing suddenly. A blood blister (subungual hematoma) from an injury appears as a dark smudge and grows out with the nail, while melanoma stays anchored at the base.
Red lines that darken slightly as they age are behaving normally. A line that started red but becomes progressively darker, wider, or irregular in color warrants a professional evaluation.