Beau’s lines are horizontal grooves or depressions that run across the fingernail, caused by a temporary pause in nail growth at the nail matrix (the tissue just beneath your cuticle where new nail cells form). When something disrupts the matrix, whether an illness, injury, or nutritional problem, it stops producing nail cells for a period. Once growth resumes, the interruption leaves behind a visible indent that slowly moves toward your fingertip as the nail grows out.
How the Nail Matrix Creates These Grooves
Your nails grow from a cluster of rapidly dividing cells in the nail matrix. Anything that temporarily shuts down or slows that cell division creates a thin, weak spot in the nail plate. As new, healthy nail pushes forward behind it, the weak spot appears as a transverse depression, sometimes shallow, sometimes deep enough to catch your fingernail on. The groove travels outward at the same pace your nail grows, roughly 3.5 mm per month for fingernails. That rate is useful: by measuring how far the line sits from your cuticle, you can estimate when the disruption happened. A groove 7 mm from the base, for example, points to an event about two months ago.
Systemic Illnesses and High Fevers
When Beau’s lines show up on multiple nails at the same level, the cause is almost always something that affected your whole body at once. Illnesses that trigger a high fever are classic culprits: scarlet fever, measles, mumps, and pneumonia can all halt nail growth long enough to leave a mark. Poorly controlled diabetes is another common trigger, likely because fluctuating blood sugar impairs the blood supply and nutrient delivery to the nail matrix. Peripheral artery disease, which narrows blood vessels and reduces flow to the hands and feet, can produce the same effect.
Any serious systemic illness, including kidney disease, liver conditions, or thyroid disorders, can temporarily suppress the rapid cell division the nail matrix depends on. The more severe the illness, the deeper the groove tends to be. In extreme cases, the interruption is so complete that the nail detaches from its bed entirely, a condition called onychomadesis, which is essentially a Beau’s line taken to its logical extreme.
Infections, Including Hand-Foot-and-Mouth Disease
Viral infections are a particularly well-documented trigger. Hand-foot-and-mouth disease (HFMD) frequently causes nail changes, including Beau’s lines, yellowish-orange discoloration, and sometimes complete nail shedding, typically appearing four to eight weeks after the initial illness. The mechanism may involve the virus directly inflaming tissue around the nail matrix, the virus replicating within the matrix itself, or the systemic stress of the infection. In most HFMD cases, the nail changes resolve on their own within about eight weeks after appearing.
Other infections, both bacterial and viral, can produce the same result if they are severe enough to temporarily suppress normal cell growth.
Chemotherapy and Other Medications
Chemotherapy drugs work by targeting rapidly dividing cells, and the nail matrix is one of the fastest-dividing tissues in the body. That makes it especially vulnerable. Beau’s lines are a typical sign of acute toxicity to the nail matrix, reflecting a transient arrest in nail production. Patients on chemotherapy cycles spaced three weeks apart often develop repeating grooves, one for each treatment round, creating a visible timeline of their therapy on the nail surface.
Chemotherapy can also produce Mees’ lines, which look different. While Beau’s lines are physical indentations you can feel with your finger, Mees’ lines are flat, white, opaque bands caused by abnormal hardening of the nail plate. Both move toward the fingertip as the nail grows, but the distinction matters: Beau’s lines reflect a pause in growth, while Mees’ lines reflect disorganized growth.
Physical Trauma to the Nail
When only one nail has a Beau’s line, the cause is usually local injury rather than a systemic problem. Common culprits include slamming a finger in a door, dropping something heavy on a fingertip, or exposure to extreme cold. Aggressive manicures and artificial nails can also damage the nail matrix enough to interrupt growth temporarily.
Chronic habits matter too. Repeatedly picking at the cuticle or skin behind the nail can cause ongoing damage to the matrix. Eczema or other inflammatory skin conditions around the nail fold can produce the same effect by inflaming the tissue where nail cells are generated. If the damage to the matrix is severe enough, the deformity can become permanent rather than growing out.
Nutritional Deficiencies
Zinc deficiency is the best-studied nutritional cause of Beau’s lines. Zinc plays a critical role in cell division, and when levels drop too low, the nail matrix can’t keep up its normal pace of production. Broader malnutrition or conditions that impair nutrient absorption can have similar effects. If you notice Beau’s lines alongside other signs of poor nutrition, such as hair thinning, slow wound healing, or frequent infections, a zinc or general nutritional deficiency is worth investigating.
One Nail Versus Many
The number of nails affected is the most useful clue to the underlying cause. A single nail with a horizontal groove almost always points to localized trauma or inflammation at that specific finger. Multiple nails showing grooves at the same distance from the cuticle suggest a systemic event, something that hit the whole body at once, like a high fever, a round of chemotherapy, or a period of severe nutritional deficiency.
The depth and width of the groove also carry information. A shallow, narrow line suggests a brief or mild disruption. A deep, wide groove indicates a longer or more severe interruption in nail growth. In most cases, Beau’s lines are temporary. As the nail continues to grow at its normal rate, the groove gradually moves to the free edge and eventually gets trimmed away, typically within six to nine months for fingernails.