Blisters are one of the leading causes of pain, and even of dropping out, over long distances such as the marathon or ultra-trail races. To avoid blisters while running, the key is to limit friction and moisture inside your shoe. Here is how to prepare your feet, choose your gear and react if a blister appears mid-race.
The short answer
- A blister forms from repeated friction, made worse by heat and moisture inside the shoe.
- The basics: shoes in your size that are already broken in, and technical socks tested in training.
- On hot spots, paper tape applied before the start reduced blisters in a trial on an ultra-trail race.
- If a blister appears, protect it with a suitable dressing rather than tearing off the skin.
Contents
Why do you get blisters when running?
A blister forms when the skin is rubbed over and over. The outer layers of the skin slide against each other, separate, and the gap fills with fluid. Heat and moisture clearly increase the risk: damp skin creates more friction than dry or very wet skin (Knapik et al., 1995). That is why blisters mostly show up on long efforts, in hot weather or after crossing streams.
With every stride, your foot takes a vertical force of around 2 to 3 times your body weight (Cavanagh and Lafortune, 1980). Repeated tens of thousands of times, this impact also explains black toenails: a bruise forms under a nail that keeps hitting the front of the shoe, especially downhill (good downhill technique in trail running limits these impacts). The nail turns purple and may eventually fall off.
Choose the right shoes and socks
First things first: shoes in the right size. Too big and your foot slides around and rubs. Too small and it is squeezed, with your toes hitting the front. Keep about a finger’s width of space in front of your big toe, since feet swell on long efforts. In wet conditions, a waterproof shoe can help on a short run; on a long effort, a breathable shoe dries faster and is often more comfortable.
Lace your shoes properly
Tighten enough that your heel does not slip, without restricting circulation: tingling or numbness is a sign you have laced too tight. On an ultra, loosen your laces a little after a few hours, once your feet have swollen.
Break in your shoes
Never race in brand-new shoes. Wear them at home first, then on several runs, including at least one long run. Same goes for socks: choose synthetic or merino wool rather than cotton, which holds moisture, and check they have no irritating seams. Double-layer socks are designed so the two layers slide against each other rather than against your skin. Studies give mixed results, but if you are prone to blisters, they are worth trying in training.
Prepare your feet before a race
Worth knowing: few prevention methods have actually been tested, and a systematic review concludes that the evidence remains limited (Worthing et al., 2017). That does not mean nothing works, but you need to test what suits you in training, never on race day.
Look after your nails and skin
Trim your toenails a few days before the race, not too short and straight across, filing the corners. About three weeks before your goal race, remove thick calluses: very hard skin can hide a deep blister underneath. Then moisturise your feet, without overdoing it in the final days so the skin does not soften.
Foot “tanning”: a field practice
Many trail runners “tan” their feet: they apply lemon juice to the clean, dry soles every morning in the weeks before their goal race, then moisturise in the evening because the acid dries the skin. The idea is to toughen the skin. It is a widespread practice, but it has not been validated by studies: if you try it, stop at any sign of irritation and only use mild products. Avoid chemical solutions such as picric acid, sometimes mentioned in old advice: it is toxic and dangerous to handle.
Protect your hot spots
This is the best-supported measure. In a randomised trial during a multi-stage ultramarathon, simple paper tape (surgical-type tape) applied to the sensitive areas of one foot reduced blisters compared with the other, unprotected foot (Lipman et al., 2015). Identify your usual friction points (heel, toes, arch) and protect them before the start. An anti-chafing cream may also help; again, test it in training.
Your feet also get used to long distances when you build up gradually. A trail running training plan that lengthens your long runs step by step prepares them better than a sudden jump in volume. If you are training for a long race, you can try RunMotion Coach free for 7 days to follow a plan built around your goal.
What to do when a blister appears
If you often get blisters, carry a few blister plasters (hydrocolloid dressings) and some tape in your pack. As soon as you feel a spot heating up, stop and protect it: a few seconds lost beats kilometres of pain. If the blister has formed, cover it without tearing off the skin, which protects the wound. Raw skin rubs even more and gets infected more easily.
At some ultra-trail races, podiatrists are available at aid stations: take a few minutes for treatment. After the race, keep the area clean and keep an eye on it. Spreading redness, warmth, pus or fever: see a health professional. For a painful black toenail or one that is lifting off, ask a doctor or podiatrist too.
FAQ: blisters and running
Should you pop a blister after a run?
A small, painless blister can simply be covered with a plaster. If it is large and bothersome, ask a health professional or podiatrist to drain it cleanly, without removing the skin.
Do double-layer socks prevent blisters?
They help some runners, but studies are mixed. Test them on several long runs before wearing them in a race.
Why do I get black toenails after a trail race?
It is a bruise under the nail, caused by repeated impacts against the front of the shoe, especially downhill. Check your shoe size and lacing.
Does lemon “tanning” work?
It is common among trail runners, but it has not been scientifically validated. Protecting hot spots with paper tape is better supported.
Feet ready for the miles
Shoes in your size and broken in, tested socks, well-trimmed nails and protected hot spots: that is the essence of avoiding blisters while running. The rest is sorted out in training, by testing your gear on long runs. To learn about other common runner problems, read our guide on how to prevent running injuries, and to prepare calmly for your next race, try RunMotion Coach for free.
Updated on 30 October 2026.
Sources: Knapik et al., Sports Med, 1995 · Cavanagh and Lafortune, J Biomech, 1980 · Worthing et al., Wilderness Environ Med, 2017 · Lipman et al., Wilderness Environ Med, 2015.
