What causes a black line in the middle of the index finger?
Nov 29, 2025
Source: Cainiu Health
Dr. Li Feng
Introduction
In general, a black line in the middle of the index finger may be caused by factors such as post-traumatic pigmentation, nail staining, melanocytic nevus of the nail, subungual hematoma, or onychomycosis. If discomfort symptoms occur, it is recommended to seek timely medical evaluation and treatment at a正规 hospital. In daily life, take care to protect fingers from injury and wear gloves when handling staining substances.
Generally, a black line in the middle of the index finger may be caused by factors such as trauma-induced pigmentation, nail staining, melanonychia (nail matrix nevus), subungual hematoma, or onychomycosis. If discomfort occurs, it is recommended to seek timely medical evaluation and treatment at a正规 hospital. Specific analyses are as follows:
1. Trauma-induced pigmentation
If the index finger has previously suffered compression or impact, capillaries under the nail may rupture. After healing, localized pigment deposition can form a black line. Avoid further injury to the nail. No special treatment is required—over time, as the nail grows, the black line will gradually move toward the tip and eventually shed.
2. Nail staining
Long-term exposure to dark substances such as hair dyes or ink, or frequent use of dark nail polish, can lead to pigment adherence on the nail surface, forming a black line. Discontinue contact with staining agents, soak the fingers in warm water, then gently wipe clean. Use mild nail cleaning products.
3. Melanonychia (Nail matrix nevus)
Abnormal pigment production by melanocytes in the nail matrix at the base of the nail leads to a longitudinal black line that grows with the nail, usually with clear borders. Monitor the width and color of the line regularly. Avoid nail trauma. Seek medical evaluation promptly if the line widens or changes color; surgical removal of the nevus may be necessary when indicated.
4. Subungual hematoma
After finger compression or impact, blood vessels under the nail rupture and bleed, causing blood to accumulate and appear as a black longitudinal stripe, often accompanied by mild swelling. Apply cold compresses within the first 48 hours to control bleeding, then switch to warm compresses after 48 hours to promote absorption of the hematoma. Avoid excessive finger strain and elevate the affected limb to reduce swelling.
5. Onychomycosis (Fungal nail infection)
Fungal infection damages the nail plate structure, leading to discoloration including black lines, often accompanied by nail thickening and brittleness. Follow medical advice for treatment with oral medications such as terbinafine hydrochloride tablets, itraconazole capsules, or topical bifonazole cream. Continue medication until a new, healthy nail fully grows in. Avoid sharing nail clippers with others.
In daily life, protect fingers from trauma; wear gloves when handling staining substances; maintain clean, dry hands to prevent fungal infections; monitor nails regularly, and consult a doctor promptly if any abnormalities occur.
