Black Toenail Explained
In brief: A black toenail, sometimes called runner's toe, is bruising under the nail caused by repeated impact of the toe against the front of the shoe. It is common after long runs, downhill running or in shoes that are too small. The nail turns dark from blood underneath and is often painless, though pressure can make it throb, and the nail may eventually loosen. Correct shoe fit, trimmed nails and good socks usually prevent it.
What is a black toenail?
A black toenail is a collection of blood beneath the nail, known medically as a subungual haematoma, caused by repeated trauma to the toe. In runners it results from the toe striking or pressing against the front of the shoe over and over during a run. The trapped blood discolours the nail, turning it red, purple, brown or black.
What causes it in runners?
The usual cause is repeated impact of the toe against the toe box, especially during long runs and downhill sections where the foot slides forward. Shoes that are too short or too tight, or laced too loosely so the foot moves around, all increase the trauma. Long toenails that catch on the shoe and thin running socks that let the foot slip can add to it.
Symptoms to recognise
The main sign is a darkening of the nail, ranging from red to black, sometimes developing over a day or two after a hard run. It is often painless, but if blood builds up under pressure it can throb or feel tender. Over the following weeks or months the affected nail may loosen and eventually come off, with a new nail growing underneath.
Treatment and recovery
Many black toenails need no treatment beyond protecting the toe and letting it grow out, which can take several months as the nail is replaced. If there is significant pain from pressure, a clinician can relieve it safely; it is best not to attempt this at home because of infection risk. Keep the area clean, and see a clinician if there is spreading redness, pus or significant pain, or if you are unsure whether the discolouration is a bruise.
Black Toenail and your running kit at OD's
Correctly fitted shoes with about a thumb's width of room at the toe, secure lacing and good moisture-wicking socks are the best prevention, keeping the toe from hammering the front of the shoe. At OD's Designer Clothing we stock premium running footwear from On, Saucony and Salomon, with fitting help in St Helens. We offer next-day delivery and free click and collect.
Please note: This guide is general information for runners, not medical advice. If you have pain that is severe, persistent or getting worse, see a GP, physiotherapist or qualified sports clinician for diagnosis and treatment.