Foot Care for Ultrarunners: Preventing Blisters on Long Runs

Evidence-based blister prevention for ultrarunners: what studies show about tape, antiperspirant, socks, shoe fit, and toenail care on long trail runs.

Blisters are not a minor inconvenience in ultrarunning. They are the single most common reason runners end up at a medical tent, and in multi-day races the numbers get ugly fast. A field study at the Al Andalus Ultimate Trail, a 219 km five-day stage race, found blisters in 34% of runners after day one, 54% after day two, and 76% after day four (Scheer et al., 2014). Toes took the worst of it, accounting for 65% of all blisters, followed by the ball of the foot at 16% and the heel at 14%.

That same study tested the usual prevention toolkit, socks, antiperspirant, powder, lubricant, and tape, and found none of it moved the needle on blister rates. The only factor that predicted fewer blisters was race experience. This is the uncomfortable starting point for any real conversation about foot care: most of what runners do to prevent blisters has weak evidence behind it, and some of it can make things worse.

This guide breaks down what the research actually supports, where the popular advice falls short, and how to build feet that hold up over 50, 100, or 160 miles.

What Causes a Blister, Mechanically

A blister forms when repeated shear force separates the layers of skin, and fluid fills the gap. It is not simply rubbing. Comaish's foundational work on friction blisters showed that the epidermis needs a minimum number of shear cycles at sufficient force before separation happens, which is why blisters tend to show up at predictable mileage rather than in the first ten minutes of a run (Rushton & Richie, 2024). Moisture accelerates this process by raising the coefficient of friction between skin and sock. Dry skin and soaked skin both blister more easily than skin at a moderate moisture level, which is why swampy trail conditions and heavy sweating both raise risk.

A 2025 systematic review of frictional dermatoses in athletes puts a number on how much distance matters: incidence rises sharply once cumulative mileage passes roughly 25 km, as sweat accumulation and repetitive shear overwhelm the skin's tolerance (Islam, 2025). That threshold lines up with where most ultrarunners start reporting hot spots on race day, and it's a useful mental marker for when foot checks should start, not after something already hurts.

The Prevention Toolkit, Ranked by Evidence

Tape: the best-supported option

Paper tape has the strongest data behind it. In the Pre-TAPED II trial, a randomized multisite study across four multi-stage 250 km races in the Jordanian, Gobi, Madagascan, and Atacama deserts, paper tape applied to blister-prone areas cut blister formation by 40% compared with untaped skin on the same foot, with a number needed to treat of just 1.31 (Lipman et al., 2016). That is a strong effect size for something this cheap. Apply tape to known trouble spots, toe webs, the ball of the foot, the back of the heel, before the skin shows any sign of irritation, and reapply once it starts to lift or bunch.

Antiperspirant: real effect, real downside

Aluminum chloride antiperspirant reduces foot sweat by more than 50% and has shown a 56% lower blister incidence in military hiking cohorts when applied on at least three separate nights before an event (Knapik et al., 1998). The catch is skin irritation. Reports of irritant dermatitis ran as high as 57% in the antiperspirant group in one trial, compared with 6% in the placebo group. If you go this route, test it during training weeks, not the night before a race, and stop immediately if the skin reddens or stings.

Powder, lubricant, and the case for testing over trusting

Foot powder has shown better moisture control than antiperspirant in some comparisons, and lubricants like petroleum jelly reduce friction directly at the skin surface. Neither has the randomized evidence tape does, and the Al Andalus data found no measurable blister reduction from any of these methods used in isolation during a multi-day race. The practical takeaway is not to abandon them, but to stop treating any single product as a guarantee. Test your combination of socks, tape, and lubricant on a long training run at race pace and race distance before you commit to it on race day.

Double-layer socks: a plausible mechanism, thinner evidence

Double-layer socks work by letting friction happen between the two sock layers instead of between sock and skin. The mechanism is sound and matches how paper tape works, by adding a low-friction surface between skin and shoe. Formal trial data is thinner here than for tape, but it is a reasonable option for runners who blister in the same spot every race and want a sock-based fix rather than a taping routine.

Fit Comes Before Any Product

No amount of tape or lubricant compensates for a shoe that does not fit the way an ultra distance demands. Feet swell over long efforts, often by half a size or more after several hours on technical terrain, which is why experienced ultrarunners size trail shoes a half to full size larger than their everyday running shoe. A thumb's width of space ahead of the longest toe when standing is the standard field check. Too little room causes toe blisters and black toenails on descents; too much room lets the foot slide and creates friction at the heel and forefoot on every stride.

This is also where a structured buildup matters more than any product. Trailia's elevation-aware training plans increase weekly vertical and time-on-feet gradually, which gives skin, not just muscles and tendons, time to adapt to sustained load. Runners who jump straight into long descents without that gradual buildup are exactly the population showing up in the blister studies above with the highest incidence.

Toenails Deserve Their Own Routine

Black toenails come from a different mechanism than most blisters, repeated impact and shear against the front or top of the shoe rather than skin-on-sock friction, and they need their own prevention plan. Trim nails straight across, several days before a long run or race rather than the night before, so any minor nicks have time to heal. Keep them short enough that they don't push into the tip of the shoe on descents, but not so short that the surrounding skin is exposed to the same pressure the nail would have absorbed. Downhill running technique matters here too: a shorter stride and a slight forward lean on descents reduces how hard the toes slam into the front of the shoe compared with braking with a long stride.

Building Feet That Hold Up Over Time

Skin adapts to repeated mechanical stress the same way muscle does, within limits. Runners with more ultramarathon experience showed measurably lower blister rates in the Al Andalus cohort, and separately, an eight-week foot-core strengthening program produced a 2.42-fold reduction in running-related injury risk in a randomized controlled trial, alongside gains in intrinsic foot muscle size (Taddei et al., 2020). That program will not directly stop a blister mid-race, but stronger, more stable feet change how load is distributed across the sole, which affects where and how often hot spots form in the first place.

Short daily foot-strengthening work, toe splays, towel scrunches, single-leg balance on uneven ground, builds this resilience over months, not days. It belongs in the same training cycle as mileage progression, not as an afterthought squeezed in during taper week.

Managing Hot Spots During a Race

The gap between a hot spot and a full blister is often 10 to 15 minutes of continued friction. The moment you feel warmth or rubbing at a specific point, stop, not later, and apply tape or a spare patch directly over the spot before it progresses. Runners who push through the first warning sign are the ones who arrive at the next aid station with a blister the size of a coin instead of a hot spot the size of a fingernail.

Carry a small kit: a few strips of pre-cut paper tape, a single-use lubricant packet, and a safety pin or sterile needle for the rare case a blister needs draining mid-race. If drainage is necessary, pierce a small edge, gently express the fluid, and leave the blister roof intact to protect the raw skin underneath. Change socks at any aid station where your feet have been wet for more than an hour; wet-to-dry sock changes are one of the few interventions every foot-care source agrees on, evidence base or not.

If nagging foot pain has been part of your last few training blocks, it's worth running that history through Trailia's Race Readiness check before you commit to a start line. A blister risk that seems manageable on a 20-mile training run can become a race-ending problem at hour 14 of a 100K, and knowing that ahead of time changes how you pack your kit and pace the day.

Key Takeaways

  • Paper tape on known trouble spots has the strongest evidence of any single intervention, cutting blister rates by roughly 40% in trial data
  • Antiperspirant reduces sweat and blister incidence but carries a real risk of skin irritation, so test it in training first
  • Shoe fit, roughly a thumb's width ahead of the longest toe, matters more than any product applied after the fact
  • Trim toenails straight across several days before long runs, not the night before
  • Treat a hot spot the moment you feel it; waiting even 15 minutes often turns a minor irritation into a race-ending blister

Conclusion

The research on blister prevention is messier than most gear marketing lets on. Tape works. Antiperspirant works but has a cost. Powder, lubricant, and double-layer socks are reasonable options without strong trial backing behind them yet. What consistently reduces blister risk across every study is the boring stuff: a shoe that fits, a training buildup long enough to condition the skin, and catching hot spots before they become blisters. Build your foot-care routine around what the evidence actually supports, test it in training, and let race day be the one time you're not troubleshooting anything new.

Citations

Scheer, B.V., Reljic, D., Murray, A., Costa, R.J. (2014). The Enemy of the Feet: Blisters in Ultraendurance Runners. Journal of the American Podiatric Medical Association, 104(5), 473-478. Lipman, G.S., Ellis, M.A., Lewis, E.J., et al. (2016). Paper Tape Prevents Foot Blisters: A Randomized Prevention Trial Assessing Paper Tape in Endurance Distances II. Clinical Journal of Sport Medicine. Rushton, R., Richie, D. (2024). Friction Blisters of the Feet: A New Paradigm to Explain Causation. Journal of Athletic Training, 59(1), 1-7. Islam, R. (2025). Cutaneous Friction Injuries and Blister Prevention in Athletes: From Stratum Corneum Mechanics to Smart-Textile Solutions. Premier Science. Knapik, J.J., Reynolds, K., Barson, J. (1998). Influence of an Antiperspirant on Foot Blister Incidence During Cross-Country Hiking. Journal of the American Academy of Dermatology. Taddei, U.T., et al. (2020). Effects of Foot-Core Training on Foot-Ankle Kinematics and Running Kinetics in Runners. Frontiers in Physiology / associated RCT data on running-related injury reduction.

Frequently Asked Questions

What actually prevents blisters in ultrarunning, according to research?

Paper tape applied to known trouble spots has the strongest trial evidence, cutting blister rates by roughly 40% in a randomized multi-stage race study. Shoe fit and a gradual training buildup that conditions the skin also matter more than most products applied after the fact. Antiperspirant works too, but it carries a real risk of skin irritation.

Does antiperspirant help prevent foot blisters?

Yes, aluminum chloride antiperspirant reduces foot sweat by more than half and has shown lower blister incidence in hiking studies when applied on multiple nights before an event. The tradeoff is skin irritation, reported in over half of users in one trial, so it needs testing during training rather than right before a race.

Why do I always get blisters on my toes during long trail runs?

Toes take the largest share of blisters in ultrarunning, around 65% in one field study, largely because of repeated toe-to-toe and toe-to-shoe friction on descents and technical terrain. Toe socks, proper shoe sizing with a thumb's width of front space, and taping the toe webs directly address this.

Should I wear two pairs of socks to prevent blisters?

Double-layer socks let friction occur between the two sock layers rather than between your skin and the sock, which is a sound mechanism similar to how paper tape works. The trial evidence is thinner than for tape, but it's a reasonable option if you blister in the same spot every time and prefer a sock-based fix.

How do I stop losing toenails during ultramarathons?

Trim toenails straight across several days before a long run or race, not the night before, so any nicks have time to heal. Make sure your shoes have enough front room, roughly a thumb's width past the longest toe, and shorten your stride slightly on descents to reduce how hard toes drive into the front of the shoe.

What should I do the moment I feel a hot spot during a race?

Stop and treat it immediately. There's often only 10 to 15 minutes between a hot spot and a full blister, so applying tape or a patch right when you feel warmth or rubbing prevents most of the damage. Waiting until the next aid station usually means dealing with a blister instead of a hot spot.

Does foot strength training reduce blister risk?

It doesn't stop friction directly, but an eight-week foot-core strengthening program has shown a 2.42-fold reduction in running-related injury risk alongside gains in foot muscle size. Stronger, more stable feet distribute load more evenly, which affects where and how often hot spots develop over a long race.

How much bigger should my trail shoes be to prevent blisters on ultra distances?

Most experienced ultrarunners size trail shoes a half to full size larger than their everyday running shoe, since feet swell noticeably during long efforts on technical terrain. The field check is a thumb's width of space ahead of the longest toe while standing, which protects against toe blisters and black toenails without leaving so much room that the foot slides.

Should I drain a blister during a race?

If a blister is large enough to affect your stride, draining can help, but it carries infection risk if not done carefully. Pierce a small edge, gently express the fluid, and leave the blister roof intact to protect the raw skin underneath rather than removing it.

When does blister risk start increasing during a long run?

Research on frictional skin injuries suggests incidence rises sharply once cumulative distance passes roughly 25 kilometers, as sweat buildup and repeated shear start to overwhelm the skin's tolerance. That's a useful point to start proactive foot checks rather than waiting until something already hurts.