Melanonychia, a black line on your nail, can be a sign of melanoma or benign pigmentation changes. Injuries, medications, and conditions like hyperthyroidism may also cause dark lines on your nails.

Not all marks on your nails are cause for alarm. "A black line that extends longitudinally from the cuticle to the free edge of the nail is often pigment," board-certified dermatologist Joshua ...

The post, shared on Reddit, featured faint black lines running down two of her nails, a symptom that can be harmless, but in ...