Horizontal ridges on your nails are almost always a sign that something temporarily interrupted your nail growth. The medical name for them is Beau’s lines: grooves or dents that run across your fingernails or toenails horizontally. Unlike vertical ridges, which are usually a harmless part of aging, horizontal ridges point to a specific event or condition that caused your nail-growing cells to briefly pause or slow down.
How Horizontal Ridges Form
Your nails grow from a cluster of cells called the nail matrix, tucked just under the skin behind your cuticle. When your body is under serious stress, whether from illness, injury, or nutritional shortage, it redirects energy toward keeping vital organs running. Nail growth is not a priority, so the matrix slows or stops producing new cells for a period. When growth resumes, the interruption leaves behind a visible dent or groove in the nail plate.
Think of it like a tree ring. The ridge marks a moment in time when something went wrong, and you can actually estimate when the disruption happened. Fingernails grow at an average rate of about 3.5 millimeters per month. If the ridge sits roughly 7 millimeters from your cuticle, the triggering event likely occurred about two months ago. This rough calculation can help you connect the ridge to a specific illness, injury, or stressful period.
Common Causes
The list of triggers is long, but they fall into a few main categories.
Illness with high fever. This is one of the most common causes. Pneumonia, measles, mumps, strep infections, and COVID-19 can all produce Beau’s lines. Your body’s inflammatory and immune response during a serious infection compromises blood flow to the nail matrix, starving it of nutrients. A systematic review of COVID-19 and nail changes found that Beau’s lines appeared an average of three months after infection and took roughly six months to fully grow out and resolve. Even COVID-19 vaccination has occasionally triggered them, with ridges appearing within days to weeks and resolving over two to four months.
Physical injury. Slamming your finger in a door, dropping something heavy on your toe, or even repeated trauma from tight shoes can damage the nail matrix directly. In these cases, the ridge typically appears on just one nail, the injured one, rather than across multiple fingers.
Chronic health conditions. Diabetes, hypothyroidism, peripheral artery disease, and Raynaud’s phenomenon all interfere with blood flow to the extremities. When circulation to the nail matrix is chronically reduced, ridges can appear on multiple nails over time. If you notice recurring horizontal ridges without an obvious illness or injury, one of these conditions may be worth investigating.
Skin conditions. Eczema and psoriasis that affect the skin around the nail can damage the matrix and produce ridges.
Nutritional deficiencies. Severe zinc deficiency and inadequate protein intake are both linked to Beau’s lines. Iron deficiency, by contrast, tends to cause vertical ridges and changes in nail shape rather than horizontal grooves.
Severe emotional stress. Divorce, the death of a loved one, job loss, and anxiety disorders can all generate enough physiological stress to pause nail growth. This surprises many people, but the mechanism is the same: your body deprioritizes nonessential functions during a crisis.
Chemotherapy and certain medications. Cancer treatment is a well-known cause, since chemotherapy targets rapidly dividing cells throughout the body, including those in the nail matrix.
Manicure damage. Long-term use of acrylic nails, gel manicures, or aggressive cuticle work can injure the matrix enough to leave horizontal dents behind.
One Nail Versus Many
The number of nails affected is one of the most useful clues for figuring out the cause. A ridge on a single nail almost always points to a local injury: something hit, crushed, or damaged that specific finger or toe. When ridges appear across multiple nails at roughly the same position, it signals a systemic event, something that affected your whole body at once, like a fever, a severe illness, or a period of extreme stress. If the ridges on different fingers sit at different distances from the cuticle, the disruption may be ongoing rather than a one-time event.
Horizontal Ridges Versus Vertical Ridges
It’s easy to confuse the two, and the distinction matters. Vertical ridges run from your cuticle toward the tip of your nail like tiny parallel lines. They’re extremely common and generally harmless, appearing more often as you age, much like fine wrinkles on the skin. Dry skin, eczema, and hypothyroidism can make them more pronounced.
Horizontal ridges are different. They run side to side across the nail and feel like a dent or groove when you run your fingertip over them. Because they result from a temporary pause in nail growth, they always have a cause, even if you can’t immediately identify it. Vertical ridges rarely need investigation. Horizontal ridges, especially on multiple nails, are worth paying attention to.
What To Expect as They Grow Out
Beau’s lines are not permanent. Once the underlying cause resolves, your nail matrix resumes normal growth and the ridge slowly moves toward the tip of your nail. At the average fingernail growth rate of 3.5 mm per month, it takes roughly four to six months for the ridge to travel from the cuticle to the free edge and get trimmed away. Toenails grow more slowly, at about 1.6 mm per month, so a ridge on a toenail can take a year or more to disappear.
During this time, the nail is structurally weaker at the groove. You may notice it catches on things or feels more fragile. Keeping nails trimmed and moisturized helps prevent the ridge from snagging or cracking as it grows out.
When Horizontal Ridges Signal Something Bigger
A single episode of Beau’s lines after a known illness or injury is typically nothing to worry about. The ridge is essentially a record of something your body already dealt with. But certain patterns deserve a closer look. Ridges that keep appearing on multiple nails without an obvious trigger may point to an undiagnosed chronic condition like diabetes, thyroid disease, or peripheral artery disease. Ridges accompanied by nail discoloration, especially white horizontal bands, can occasionally indicate toxin exposure, including arsenic poisoning. And if the ridges are deep enough that the nail nearly separates from the nail bed, the underlying disruption was significant and warrants evaluation by a healthcare provider.
If you recently went through a high fever, a COVID-19 infection, a major surgery, or an intensely stressful period, the ridges appearing a few weeks to months later are most likely the expected aftermath. They’ll grow out on their own and won’t come back unless the disruption happens again.