Common Running Irritations: Blisters, Chafing, Black Toenails and More

Not every ache that comes with running is a warning sign. Alongside the more serious overuse injuries — the kind covered in RaceRove's guide to runner's knee and shin splints — there's a whole category of smaller, near-universal annoyances that almost every runner deals with sooner or later: a blister after a new pair of shoes, some chafing on a long run, a toenail that goes black and eventually falls off, a side stitch that shows up out of nowhere. None of these are injuries in the usual sense. They're mostly friction, fit and a body still adapting to load, and almost all of them are preventable once you know what causes them.

This guide covers the smaller stuff: what typically causes it, how to prevent it, and what basic self-care looks like when it happens anyway. If something you're feeling sounds more like real pain than surface irritation — sharp, sudden, worsening, or not settling down after a few days — that's a different conversation, and the runner's knee and shin splints guide, or a doctor, is the better place to start.

Blisters

A blister is the body's response to repeated friction: skin rubbing against a sock or shoe in the same spot, over and over, until fluid collects between layers of skin to cushion the irritation. It's rarely about the shoe or the sock alone — it's about movement between skin and fabric that a well-fitted setup mostly eliminates.

• Shoes that don't fit well. Too loose and the foot slides around inside; too tight or too short and toes jam against the front on every step, especially downhill. Both create the sliding that starts a blister.
• Wet or sweaty socks. Damp skin softens and grips less well, so it moves more against the sock — one reason blisters show up more on hot days and longer runs.
• A new shoe that hasn't been broken in. Stiffer materials haven't yet softened to the shape of your foot, so they tend to rub in spots a broken-in shoe wouldn't.

• Socks built for running. A moisture-wicking synthetic or wool blend moves sweat away from the skin far better than cotton, which stays wet and heavy once it's damp.
• Lubricant or tape on known hot spots. If a certain spot on your heel or toe blisters every time, treat it before the run — a thin layer of lubricant or a strip of athletic tape there is cheap insurance.
• Breaking in new shoes gradually. A few shorter runs before race day lets the upper soften and lets you catch a hot spot while it's still minor.

If a blister does form, the usual approach is simple: keep it clean, cover it with a bandage or blister-specific dressing so it stops rubbing further, and leave a small one to heal on its own rather than picking at it. A blister that's unusually large, keeps refilling, or starts looking red, warm or infected is worth a doctor's attention rather than home remedies.

Chafing, including nipple chafing

Chafing works the same way a blister does — skin rubbing against fabric or skin — just spread across a wider area instead of concentrated at one point. It shows up wherever there's repeated contact and movement: the inner thighs, underarms, a waistband where a belt sits, sports-bra lines, and, for both men and women on longer runs, the chest.

Nipple chafing is extremely common on longer runs and nothing to feel awkward about — it happens from the same friction as anywhere else, just against fabric that's moving with every stride. It's also one of the easiest items on this list to prevent outright: a dab of lubricant or a couple of pieces of athletic tape before a long run stops it before it starts, and most runners who deal with it once simply build that step into their pre-run routine from then on.

• Lubricant anywhere that rubs — thighs, underarms, waistband, chest — applied before the run starts, not after it's already uncomfortable.
• Moisture-wicking, seam-friendly clothing. Fabric that stays dry moves more smoothly against skin than fabric that's wet and clinging, and flat seams irritate less than raised ones over a long run.
• Tape on repeat problem spots. If the same area chafes every long run, it's worth taping it beforehand rather than re-solving the same problem afterward.

Chafed skin is treated much like any other irritated skin: clean it gently, keep it dry, and give it a day or two before your next long run in the same spot. It typically settles down quickly once the friction stops.

Black toenails

A black toenail happens when a toe repeatedly strikes the front of the shoe, usually the big toe or the one next to it, and small blood vessels under the nail bleed slightly from the repeated impact. It shows up more on downhill sections, where the foot slides forward inside the shoe with every step, and in shoes that run slightly too small or too narrow through the toe box.

This is largely a sizing and fit issue rather than anything about running technique. A commonly given piece of advice is to go up about half a size from your regular casual shoe size when buying running shoes, since feet swell over the course of a run and need a little extra room at the front — particularly for anything downhill, or long enough that swelling becomes noticeable.

A black toenail usually looks worse than it is: it's typically painless once it's happened, and the nail grows out and gets replaced over the following months without any treatment needed. A toenail that's genuinely painful, throbbing, or shows signs of infection — spreading redness, warmth, or discharge — is worth a doctor's look rather than waiting it out.

Side stitches

A side stitch is a sharp, cramping pain, usually felt just under the ribs on one side, that turns up mid-run and can be sharp enough to make you slow down or stop. It's uncomfortable rather than dangerous, and it tends to ease once you address whatever triggered it.

• Eating too close to a run. A stomach still digesting a meal, especially something large or high in fat, is a common trigger — most runners find that leaving more time between eating and running helps.
• Shallow breathing, often a side effect of starting faster than your fitness supports, which limits how well the diaphragm can work.
• Starting too fast in general. A stitch often shows up in the first mile or two of a race, when adrenaline and excitement push the pace above what a proper warm-up would suggest.

• Slow down. Easing the pace, even briefly, is usually the fastest way to settle a stitch — pushing through it tends to make it worse.
• Press on the area with your fingers while you continue to run or walk; gentle pressure on the spot is a commonly used trick that seems to help many runners.
• Exhale deliberately as the opposite foot lands. If the stitch is on your right side, focus on breathing out fully as your left foot strikes the ground, and the other way around — changing the rhythm between breath and stride is a widely used fix.

IT band irritation — one worth knowing the name of

The iliotibial band, or IT band, is a thick band of connective tissue running down the outside of the thigh from the hip to just below the knee. It doesn't usually cause trouble, but when it's aggravated it produces a sharp or burning pain on the outside of the knee, and it's common enough among runners that it's worth knowing by name.

• A sudden increase in mileage — the same pattern behind most overuse irritation in running: tissue asked to do more before it's adapted.
• Worn-out shoes, which lose cushioning and support in ways that change how load travels up the leg well before they look obviously worn.
• Always running the same direction on a cambered road. A road's slope tilts the same hip and knee the same way on every stride if you never switch sides or run the loop the other way.

General self-care follows the same pattern as other overuse irritation in running: relative rest rather than stopping entirely, foam rolling the outside of the thigh, and hip strength work, since a good deal of IT band irritation traces back to weak hip muscles rather than the band itself. RaceRove's runner's knee and shin splints guide covers this same relative-rest-and-strength-work approach in more depth, since the underlying pattern is so similar. If it doesn't ease up within a couple of weeks of backing off, that's a reasonable point to see a physical therapist rather than continuing to guess.

What all of this has in common

Blisters, chafing, black toenails, side stitches and IT band irritation look like a random list of complaints, but they trace back to the same handful of root causes: friction between skin and fabric or shoe, a fit that doesn't quite match your foot or body, and tissue that's still adapting to a training load. None of them is usually a reason to stop running altogether — each is a reason to adjust something small.

Of everything on this list, a properly fitted, broken-in shoe does more to prevent it than almost any other single change. Sizing that leaves room at the toe box heads off black toenails and cuts blister risk; a shoe that still has its cushioning intact protects against the load-related irritation behind IT band trouble; and a pair you've actually broken in before race day removes one of the most common blister triggers outright. If you're not sure whether your current pair is still doing its job, RaceRove's guide to replacing your running shoes walks through the signs it's time for a new pair.

None of this should keep you from lining up. Most runners deal with a version of this list at some point and keep running anyway — a little preparation just makes it a non-issue on race morning. Whether you're training for a first 5K, stepping up to a 10K, chasing a half marathon PR, or building toward a full marathon, the fixes above travel with you. Browse the full list of races by state when you're ready to pick the next one.