Dark lines or spots under nails can arise from injuries, like slamming a finger in a door, causing bleeding beneath. Yet, if dark streaks appear without trauma, rush to a doctor, as this could signal skin cancer. Known as melanonychia, these streaks are particularly alarming if isolated to one nail. While some lines may be benign, tied to medication, pregnancy, or ethnicity, any new or evolving mark warrants a check by a medical professional. Subungual melanoma, a dangerous form of skin cancer, is the gravest risk. Early detection is vital, so any unexplained dark line—especially if it widens or darkens—needs immediate evaluation.
