Dark spots or lines under the nails can result from injuries, such as jamming your finger, which may cause bleeding under the nail. However, if you notice dark streaks without any recent trauma, seek medical attention promptly, as this could indicate skin cancer. Known as melanonychia, these streaks are particularly concerning if they appear on a single nail. Some lines are benign and may be related to medications, pregnancy, or ethnic background, but any new or changing line should be evaluated. Subungual melanoma, a serious form of skin cancer, is the most critical concern. Early detection is crucial, so any unexplained dark line—especially one that expands or darkens over time—should be examined without delay.
