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Will running turn my toenails purple?

Purple and black toenails are a common sight on runner’s feet. What causes this pedicure nightmare, and is it something to be worried about?

About two weeks ago, hundreds of runners lined up at Parc Jean Drapeau to run the Montreal 21K race. But beneath neon-coloured running shoes and Nike sports socks, you would be surprised to learn how many of these runners have black toenails or are missing a toenail completely.

Runner’s toenail, tennis toe, black toenail – all these names refer to the same phenomena. A deep purple or black toenail, usually a bit tender but not very painful, that sometimes falls off. You don’t have to be a world-class marathon runner to get a black toenail. Runner’s toenails are quite common – I experienced this unusual surprise after a long run and know many friends who have lost a toenail or two. Should I be concerned if a toenail turns purple? Or is this condition more pedicure ailment than medical worry?

Before diving into the science behind purple toenails, we first need to know what a healthy toenail consists of. Nails are mostly made of a protein called keratin – the same protein that is the basic constituent of our hair, horses’ hooves, and dogs’ claws. A healthy nail is attached to our fingers and toes at the nail bed. Our bodies consistently produce new keratin from the nail matrix, a specialized layer of cells located at the base of our nails. The new keratin pushes the existing nail out, which is why we need to clip and trim our nails every week or so. If you look at the base of your nail, you’ll see a faint half-moon shape. This is called the lunula (luna is moon in latin) and is the visible portion of the nail matrix! Nails help protect our fingers and toes, while allowing us to scratch, grab, and pick up items more easily.

The medical term for runner’s toenail is subungual hematoma. Subungual means ‘below nail’ and hematoma means ‘pool of blood’. When running, toes will bump against the top or side of the running shoe. Repeated bumping over long distances can cause the blood vessels under the toenail to burst, leading to a hematoma forming under the nail. Have you ever dropped something heavy on your toe and had your toenail turn purple afterwards? The same can happen with running, except spread over time and distance. Like dropping something small on your toe many times! The effect adds up and leads to the same outcome – a subungual hematoma.

When runners see a deep purple or black toenail after a long week of training, they are actually seeing a pool of blood forming in the skin underneath the nail. Sometimes, if the hematoma is big enough, it will put pressure on the above nail and separate it from the nail bed. This will cause the nail to fall off, although it can take weeks or months. Sometimes you’ll find a ‘mini nail’ underneath the fallen-off toenail, because the nail matrix already started producing keratin under the detached toenail.

Despite being less than ideal for sandal season and beach vacations, runner’s toenail is generally harmless. Minimizing the bumping of your toes by wearing good quality running socks and well-fitting shoes can help prevent future purple toenails. However, if you have a lot of pain, or are concerned about the size of the hematoma, see a physician. It’s better to be safe than sorry, especially if you’d like to stay on your feet far into the future.


Maya McKeown recently graduated from McGill University with a Bachelors of Science (BSc) in Neuroscience and a minor in Mathematics.

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