Ingrown toenails happen when the edge of a nail grows into the soft skin alongside it, triggering pain, swelling, and sometimes infection. The big toe is the most common site. While the immediate cause is always the same, a nail edge digging into the surrounding tissue, several factors determine whether this actually happens to you.
How the Nail Penetrates the Skin
The leading explanation is straightforward: the edge of the nail plate grows into the overlapping fold of skin beside it. Once the nail breaks through, the body treats it like a foreign object, launching an inflammatory response that causes redness, swelling, and pain. If the irritation continues, the body produces granulation tissue, the raw, bumpy tissue you might notice forming around the affected corner of the nail.
A competing theory focuses on the skin itself rather than the nail. In some people, the soft tissue alongside the nail is naturally wider or puffier, causing it to bulge up and press against the nail edge. Over time, constant pressure between the skin and the nail creates damage and breakdown of the tissue. In practice, both mechanisms likely work together: the nail pushes outward while the skin presses inward, and anything that increases force on either side raises your risk.
Trimming Your Nails Incorrectly
This is the single most common preventable cause. Two mistakes account for most trimming-related ingrown nails. The first is cutting nails too short. When a nail is trimmed below the tip of the toe, the skin at the edge can fold over the nail as it regrows, guiding it into the flesh instead of over it. The second mistake is rounding the corners. Many people curve the edges to match the shape of their toe, thinking it looks neater, but this removes the straight edge that normally sits on top of the skin fold. Without that flat edge, the nail’s corner can angle downward and pierce the skin as it grows out.
Podiatrists recommend cutting straight across and leaving about 1 to 2 millimeters of nail beyond where it attaches to the toe. That small margin keeps the nail long enough to grow over the skin rather than into it.
Tight or Narrow Shoes
Footwear is the other major external cause. Shoes with a narrow or pointed toe box cramp the toes together, creating constant lateral pressure that can force the nail sideways into the skin. This is especially true for the big toe, which bears the most contact with the inside of the shoe.
High heels compound the problem. Elevating the heel shifts body weight forward, pushing the toes into the front of the shoe. The combination of downward force and a tight space significantly increases pressure on the toenails. Rigid shoes made from stiff materials add another layer of risk because they don’t flex with the foot’s natural movement, creating persistent pressure points around the nails throughout the day.
Sports and Repetitive Foot Trauma
Athletic activity increases the forces that drive nail into skin. Every time your foot strikes the ground during running, the reactive force pushes back up through your toes. A study of young male runners found that 74.1% of them had ingrown toenails, a dramatically higher rate than in non-athletes. The same study found that runners were far more likely to have hard, thick nails (74.4% of runners versus 25.6% of non-athletes), and that harder nail consistency was directly linked to higher ingrown nail rates.
Any sport that involves sudden stops, kicking, or repeated toe impact can have similar effects. Soccer players slam their toes into the ball and the ground. Dancers, especially in ballet, compress their toes into narrow pointe shoes for hours. Even hiking in boots that allow the foot to slide forward on downhill stretches can repeatedly jam the toenail into the surrounding skin.
Nail Shape You Inherit
Some people are simply born with nails that curve more sharply than average. Pincer nails are the most extreme example: the nail progressively curves inward from base to tip, creating a tube-like or trumpet shape at the free edge that digs into the skin on both sides. This trait can be inherited in an autosomal dominant pattern, meaning if one parent has it, there’s a roughly 50% chance of passing it on. People with hereditary pincer nails typically see the same deformity on both feet.
Three variations exist. An omega nail arches into an almost circular cross-section. A plicated nail curves more sharply on one side than the other, causing ingrowth mainly on that side. A tile-shaped nail has sharply bent lateral edges while the center stays relatively flat. Even without a full pincer deformity, naturally wide or fan-shaped nails increase risk because there’s more nail edge in contact with the surrounding skin.
Body Weight and Excess Skin
Obesity increases the ground reaction forces your feet absorb with every step, which pushes the nail edge harder into the surrounding tissue. At the same time, excess soft tissue around the toes can bulge over the nail borders, creating the same pinching effect as tight shoes but from within. The combination of greater force and more tissue pressing against the nail makes ingrown toenails considerably more common in people carrying extra weight.
Fungal Infections and Thickened Nails
Fungal nail infections change the nail’s structure. An infected nail often becomes thicker, more brittle, and irregularly shaped. As the nail distorts, its edges are more likely to press unevenly into the skin fold. Thickened nails are also harder to trim cleanly, which makes it easier to leave jagged edges or uneven corners that catch on the surrounding tissue.
What Happens If It Gets Worse
An untreated ingrown toenail typically progresses through recognizable stages. It starts with tenderness, mild swelling, and redness along one side of the nail. If the nail continues to dig in, the area becomes infected: you may notice increased pain, warmth, pus, and more pronounced swelling. At its most advanced stage, the body builds up granulation tissue over the nail edge, and the infection can spread beyond the toe.
In rare cases, the infection can progress to cellulitis, a spreading skin infection, or even osteomyelitis, an infection of the underlying bone. These complications are more common in people with diabetes or poor circulation, whose immune response in the feet is already compromised. Persistent pain, red streaks extending away from the toe, or a fever are signs the infection has moved beyond a simple ingrown nail.
Reducing Your Risk
Most ingrown toenails are preventable with a few habits. Cut your nails straight across rather than rounding the corners, and don’t trim them shorter than the tip of the toe. Use clippers large enough to cut across the full width of the nail in one or two passes rather than nibbling at the edges. Choose shoes with a roomy toe box that lets your toes lie flat without pressing against each other or the sides of the shoe. If you run or play sports with heavy foot impact, make sure your athletic shoes fit properly, with about a thumb’s width of space between your longest toe and the front of the shoe.
If you notice an ingrown nail early, soaking the foot in warm water several times a day can reduce swelling and allow you to gently lift the nail edge away from the skin. Placing a small piece of clean cotton or dental floss under the corner of the nail can help redirect growth outward. For nails that keep coming back or show signs of infection, a podiatrist can remove the offending nail border and, if needed, treat the nail root to prevent that edge from regrowing.