Dark lines or marks under the nails can result from trauma, such as pinching your finger in a door, causing bleeding beneath the nail. However, if you notice dark streaks without any injury, it is important to see a doctor immediately, as this can be a sign of skin cancer. Medically known as melanonychia, these dark streaks are especially concerning if they appear on a single nail. While some lines can be harmless and related to medication, pregnancy, or ethnicity, any new or changing line should be checked by a healthcare provider. Subungual melanoma, a type of skin cancer, is the most serious concern. Early detection is crucial, so any unexplained dark line—especially one that gets wider or darker—should be evaluated promptly.
