Hami Mahani

How to Prevent Blisters and Chafing During a Marathon

Blisters and chafing are preventable: the shoes, socks, skin prep, clothing, and in-race hot-spot fixes that keep friction from stealing your marathon finish.

Your training is done. Your carb load is planned. Then at mile 18 a tiny hot spot on your heel turns into a blister, your shirt starts sawing at your chest, and every step becomes a negotiation. Blisters and chafing rarely end a marathon outright — but they steal the finish you trained for, one painful step at a time.

The good news: both are almost entirely preventable. They are not bad luck — they are friction, moisture, and heat building up over thousands of steps, and you can control all three.

Why Blisters and Chafing Actually Happen

A blister is skin shearing — layers pulling apart under repeated rubbing, then filling with fluid. Three things make it worse: friction (skin vs. shoe, sock, or seam), moisture (sweat softens skin), and heat (warm feet swell, increasing pressure).

Chafing is the same story on thighs, underarms, chest, and waistband. Salt from dried sweat adds grit, and a seam fine at mile 3 can feel like sandpaper at mile 20. Rule for everything below: test it on a long run, never on race day.

Step 1: Start With Your Shoes and Socks

Fit matters more than brand. Your feet swell during a marathon, so most runners need a thumb's width of space in front of their longest toe and a shoe they have already run long in. If a shoe gives you a hot spot at 14 miles in training, it will not magically behave at 22.

Use the heel lock. That extra eyelet at the top of almost every running shoe is there for a reason. Lacing through it (often called a runner's loop) holds your heel in place and stops your foot sliding forward into the toe box — the slide that causes black toenails and forefoot blisters.

Ditch cotton socks. Cotton holds sweat against skin. Choose synthetic or merino-wool running socks, snug with no fabric bunching. Some runners swear by toe socks to stop toe-on-toe rubbing; others prefer double-layer socks. Honestly, both work — the best sock is the pair you tested on your longest run with zero hot spots.

Step 2: Prep the Skin Before It Becomes a Problem

On race morning, cover every known trouble spot before you start:

  • Anti-chafe balm or petroleum jelly on thighs, underarms, chest, waistband line, and sports-bra or heart-rate-monitor edges. Reapply at aid stations if you know you are a heavy sweater — many races hand out sticks for exactly this.
  • Tape or blister patches on heels, the balls of your feet, or any toe that has blistered before. Apply to clean, dry skin before you put socks on, and round the corners so edges do not peel.
  • Trim toenails a week out — not the night before. Too long, they press and rub; cut too short the day before, they create a brand-new sensitive edge.

This whole routine takes five minutes. It belongs in your day-before marathon checklist and your race-morning plan, right next to pinning your bib and laying out your fuel.

Step 3: Dress for Mile 20, Not Mile 1

Choose smooth, moisture-wicking fabrics with flat seams. Shorts liners that ride up, a loose shirt that swings, or a brand-new singlet are all race-day risks.

Do a full dress rehearsal on an 18- to 20-miler — same shoes, socks, shorts, top, and belt. Your shakeout run is too short to reveal these problems. If something rubs in training, change it then. Same principle as training your gut: nothing new, everything rehearsed.

Rain makes everything worse — wet skin chafes faster and wet socks blister quickly. On a wet day, add more balm and plan to reapply at halfway.

Step 4: Act on the First Hot Spot

Experienced runners still make this mistake: feeling a hot spot at mile 10 and thinking, "I'll deal with it later." Later, it is a blister.

A hot spot is a warning, not an injury. If you can, stop for 30 seconds: smooth out your sock, retie your shoe slightly tighter or looser, add balm from an aid station, or cover the spot with tape from your belt. Thirty seconds at mile 10 saves ten minutes of limping at mile 24. Think of it like fueling and pacing a marathon — small, early corrections beat heroic late ones.

That is also why I personally prefer fuel I can handle cleanly on the move. Sticky hands from gels, wiped on shorts and skin, do not help a chafing problem. On my long runs I use our Marathon Gummies for a lot of my fueling because I can pop them and keep running — no sticky mess on my hands, no mess on my gear. It is a small thing, but at mile 20, small things are the whole race.

If You Still Get a Blister: Race Day and After

If a blister forms during the race and it is not changing your stride, leave it alone and finish. If it is forcing you to run differently — which risks your knee, hip, or other foot — stop at a medical tent and let them dress it properly.

After the race:

  • Leave intact blisters alone when you can. The fluid protects the new skin underneath. Cover with a clean blister patch and keep it dry.
  • If a blister is large or too painful to walk on, clean it first; if you drain it, use a sterilised needle at the edge, keep the skin roof in place, then cover it. Redness, warmth, or streaking means get checked.
  • For raw chafing, rinse gently (the shower will sting — everyone learns this once), pat dry, and use a plain protective ointment while the skin calms down. Loose clothing for a day or two helps more than any fancy cream.

Our guide to recovering after a marathon in your first 7 days covers the full picture. Personally, after finish-line food and water, I use FinishLine as part of my recovery routine — getting aminos, protein, and fluids in early is one less thing to overthink when everything hurts.

Your 7-Day Blister-Proofing Countdown

  • 7 days out: Trim toenails. Confirm your exact race shoes and socks. No new gear purchases for race day.
  • In training: Full kit on your long run; note every rub point.
  • 2 days out: Lay out balm, tape, and patches with your bib and fuel — see our taper week nutrition guide for the food side of the same week.
  • Race morning: Balm every trouble spot, tape known hot spots, heel-lock your laces, smooth your socks flat.
  • During the race: Act on the first hot spot. Reapply balm at aid stations if you are prone to chafing.
  • After: Clean, cover, and let the skin recover before your next run.

A little preparation — right socks, five minutes of skin prep, one honest dress rehearsal — removes blisters and chafing from the race entirely, so the only thing left to think about is running it.

Your turn: What is your worst blister or chafing spot — heels, toes, thighs, or somewhere else — and what finally fixed it for you? Tell us in the comments; the best fixes always come from other runners.

— Hami, 6AM Run