Runners’ feet can be pretty gnarly. Between black or bruised toenails, blisters, calluses, and athlete’s foot, almost every runner would be a good candidate for a pedicure any day of the week.
A blister under your toenail is one of the many common foot maladies you may contend with as a runner. Most of the time, a black toenail actually results from a blood blister under your nail from running, which may or may not pop and spread the blood under the entire toenail.
If you’re in the large group of runners desperately seeking quick advice and best practices for “a blister under your toenail from running,” rest assured we have you covered.
In this guide, we will discuss how you get a blister under your toenail from running and how to treat a blister under your toenail to ensure you don’t miss out on training.
We will look at:
- What Does A Blister Under Your Toenail From Running Look Like?
- What Causes A Blister Under Your Toenail?
- Factors That Increase the Risk of A Blister Under Your Toenail from Running
- How to Treat A Blister Under Your Toenail From Running
- Preventing A Blister Under Your Toenail from Running
Let’s get started!
The Honest Truth About A Blister Under Your Toenail
The medical name is subungual hematoma, and it’s usually not a friction blister at all — it’s blood pooled between the nail plate and the nail bed from repetitive impact. The mechanism, the right treatment, and the conditions that cause it differ enough from skin blisters that the popular conflation between “toenail blister” and “blister” produces the wrong fixes.
Subungual hematoma: the impact mechanism
Subungual hematoma forms when repetitive shear or impact ruptures small vessels in the nail bed and blood collects under the rigid nail plate. In runners, the dominant mechanism is the toe sliding forward in a too-short or too-loose shoe and slamming the toenail into the toe-box ceiling on each downhill stride or hard footstrike 1Adams BB. Skin and sport. Phys Sportsmed. 2002;30(11):27-37.. Mailler and Adams’ dermatology survey of marathon finishers documented subungual hematoma in 19–31 percent of runners on race day, with the great toe (hallux) and second toe most affected, and rate rising with distance and downhill running 2Mailler EA, Adams BB. The wear and tear of 26.2: dermatological injuries reported on marathon day. Br J Sports Med. 2004;38(4):498-501.. The pressure from accumulated blood is what produces the throbbing pain — the nail plate doesn’t expand, so even a small volume of blood generates substantial pressure inside the nail-bed compartment.
Foot swelling and shoe sizing: the upstream fix
The single most common upstream cause is shoe sizing that doesn’t account for foot-volume increase during running. Foot volume rises 2–6 percent in 30–60 minutes of running and 8–10 percent in hot, humid conditions or longer runs 3Stolwijk NM, Duysens J, Louwerens JW, Keijsers NL. Plantar pressure changes after long-distance walking. Med Sci Sports Exerc. 2010;42(12):2264-72.. The half-size-up rule (or roughly 1 cm forefoot space measured from longest toe to shoe end) reflects this swelling reality — shoes that fit snug at the start of a run will preferentially fill the toe-box as feet swell, with the nail-impact region taking the load 4Branthwaite H, Chockalingam N, Greenhalgh A. The effect of shoe toe box shape and volume on forefoot interdigital and plantar pressures in healthy female footwear consumers. J Foot Ankle Res. 2013;6(1):28.. Heel-lock lacing patterns that anchor the heel back into the heel counter are the most under-used intervention; they prevent the foot from sliding forward into the toe-box during downhill phases or late-race fatigue, reducing nail-impact significantly 5Hagen M, Hennig EM. Effects of different shoe-lacing patterns on the biomechanics of running shoes. J Sports Sci. 2009;27(3):267-75..
Treatment: when to drain and when to leave it alone
Painful subungual hematomas with substantial blood (covering more than approximately 25 percent of the nail bed) benefit from prompt decompression via nail-plate trephination — making a small hole through the nail plate to drain the blood and relieve pressure. Done within 24–48 hours of formation, this is highly effective at relieving pain and largely preserves the nail; done later, the blood clots and the nail will typically detach 6Salter SA, Ciocon DH, Gowrishankar TR, Kimball AB. Subungual hematoma. Cutis. 2006;78(2):103-7.. Small, painless hematomas usually resolve on their own and don’t require intervention. Trephination should be done under sterile conditions to avoid introducing infection through the nail plate, ideally by a clinician; do-it-yourself attempts with unsterile tools introduce infection risk that exceeds the value of self-drainage 7Skayem C, Saturnino C, Gilliet M, et al. Nail trauma management: a comprehensive review. Int J Dermatol. 2022;61(8):925-32.. The nail typically detaches and falls off over 4–8 weeks regardless of treatment in larger hematomas; a new nail grows in over 4–6 months for the great toe.
When the “blister” isn’t a hematoma at all
Some toenail-area blisters are actual friction blisters at the lateral or medial nail fold rather than subungual hematomas. The diagnostic difference matters: friction blisters at the nail fold respond to anti-chafe lubricants, double-layer socks, or moleskin patches, while subungual hematomas need shoe-fit changes and lacing fixes. Friction blister biology was characterised by Knapik and colleagues: damp skin produces the highest friction at skin-textile interfaces, with fully wet skin and dry skin both lower — the implication is to either fully dry or fully lubricate the area, not leave it in the damp middle 8Knapik JJ, Reynolds KL, Duplantis KL, Jones BH. Friction blisters: pathophysiology, prevention and treatment. Sports Med. 1995;20(3):136-47. 9Quinn EK, Massey PA, Cain MT, et al. Friction blisters in athletes: pathophysiology, prevention, and management. Curr Sports Med Rep. 2019;18(4):127-31.. The visual cue: subungual hematoma sits underneath the nail plate (you see purple-black through the nail), while friction blister sits at the soft tissue around the nail.
When to see a clinician
Three situations push subungual hematoma into “clinician-please” territory. First, severe pain that doesn’t respond to elevation, ice, and OTC analgesics within 24 hours suggests pressure that warrants drainage rather than waiting. Second, signs of infection — warmth, spreading redness, fever, drainage, lymphangitic streaks — indicate cellulitis or paronychia and need antibiotic treatment. Third, suspected fracture: hematoma covering more than 50 percent of the nail bed has historically been considered a marker for underlying distal phalanx fracture, though more recent imaging studies have moderated this rule 10Roser SE, Gellman H. Comparison of nail bed repair versus nail trephination for subungual hematomas in children. J Hand Surg Am. 1999;24(6):1166-70.. Recurrent toenail hematomas in the same toe across multiple race cycles often indicate a structural mismatch between the runner’s foot and their preferred shoe last; a foot-typing exam at a specialty running shop or with a podiatrist resolves more cases than another shoe purchase 11Branthwaite H, Chockalingam N. Everyday footwear: an overview of what we know and what we should know on ill-fitting footwear and associated pain and pathology. Foot. 2019;39:11-4.. The honest reading: subungual hematoma is mostly a fit-and-lacing problem with a clean medical fix when the pain warrants it.
What Does A Blister Under Your Toenail From Running Look Like?
We’ve all seen a blister at one time or another. They usually look like small, fluid-filled sacs with clear fluid inside the bubble formed by the outer layers of skin.
The clear fluid inside a regular blister is called serum.
Blisters under toenails from running can take on a couple of different forms.
Some runners get the classic clear bubble blisters at or around the edges of a toenail. They will look like pretty much any blister you’d see on your body; only the blister will be at least partially under the toenail.
The edge of the nail where you trim it shorter may be slightly raised or pulling up away from the nail bed underneath, and you might notice puffiness in the area of the blister where it extends to your skin in the cuticle or outside of your toenail.
Some blisters under toenails are blood blisters, which means that the fluid inside the blister is blood instead of clear serum.
The entire toenail may appear black, blue, or bruised, depending on the size, shape, and status of the blood blister, or there may be a distinct blood blister under your toenail where just that region is either dark red, blue, purple, or black.
In the latter case, the blood blister under your toenail has not yet ruptured.
Blood blisters under the toenails from running are actually so common that they’ve earned a nickname—Runner’s Toe—which is often used in common parlance in place of the medical term, subungual hematoma.
What Causes A Blister Under Your Toenail?
Regardless of where they are on your body, most blisters are caused by friction against the skin, and blisters under the toenail can be caused by friction as well.
Even though it’s hard to imagine that there would be friction under a toenail, there actually can be friction or shearing of the skin under the toenail—called the nail bed—relative to the toenail on top.
Toenails are attached to the nail bed, which is the fragile skin underneath the toenail.
When you run, if your toenails bump up against the front inside edge of your running shoe or the top of the shoe, the toenail can get pushed towards your body, which moves the toenail and nail bed skin towards you.
However, the bones in your toes are moving forward as you run. This results in a stretching and shearing force on the fragile nail bed skin from the toenail and the toe, moving in opposite directions.
As your steps accrue while you run, this repetitive shearing and stretching can cause a blister under your toenail.
Factors That Increase the Risk of A Blister Under Your Toenail from Running
In most cases, you can prevent blisters under your toenails from running by wearing the right footwear, but there are additional risk factors that can make you more prone to getting a blister under your toenail.
The following factors can increase the risk of getting a blister under your toenail when you run:
- Wearing running shoes that are too small/short
- Wearing running shoes that are too narrow in the toe box or do not have enough height in the toe box around your feet
- Wearing running shoes that are too big, which allows your foot to slide around
- Running with toenails that are too long
- Running in damp socks or socks that don’t wick moisture
- Claw toes or hammer toes, which are toes that are severely curled or bent so that you bear weight on the tips of your toes (in which case, you may have blisters on the tips of your toes and under the top edge of your toenail)
- Excessive downhill running
- Thick or fungus-infected toenails
How to Treat A Blister Under Your Toenail From Running
The best way to treat a blister under your toenail from running depends on the location of the blister.
For clear blisters or blood blisters that are around the tip of your toe or edges of your toenail, it’s best to leave the blister alone and allow it to run its course.
However, if the blister is bothering you, you can take a sterilized needle and pop the blister to release the fluid and reduce the pressure underneath your toenail.
Then, if possible, soak your foot in warm salt water for 15 minutes. Fully dry your foot, and then apply antibacterial ointment and a sterile bandage.
If the edges of the blister are fully confined under the toenail so that you can’t reach a sterile needle into the blister to release the fluid, you’ll need to drill a hole through your nail.
This is best done by a medical professional, though some runners do DIY toenail drilling with sterile hypodermic needles. We don’t necessarily recommend this practice.
Alternatively, although you won’t get the same immediate release you would get some lancing a blister under your toenail, it can be helpful to soak your foot in warm salt water when you have a blood blister.
This can soften the skin and reduce the risk of infection.
Preventing A Blister Under Your Toenail from Running
Ultimately, the old adage, “An ounce of prevention is worth a pound of cure,” certainly applies here.
The best way to prevent blisters under your toenails is to keep your toenails even and short with clean edges and to wear running shoes that fit properly.
You want your shoes to have enough length so that your toenails aren’t hitting the end of the shoes, especially when you run downhill, but not so big that your feet are moving around in the shoes.
Work with a shoe fit expert at your local running shoe store to find the right size, and consider an alternate lacing pattern to keep your feet from moving around in the shoes as you run.
Finally, vary your terrain and avoid excessive downhill running. Repetitive motion increases the risk of blister formation, and downhill running increases the shearing between your toenails and nail bed.
Have you had a blister under your toenail? Let us know how you handled it!
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