Bruised toenails are often viewed as a marker of the long-distance runner. At some point in the career of most runners, they will slide their shoe off after a long run to find that one toenail turned black. The good news about this affliction is that the extent of the damage is aesthetic as black toenails typically don't hurt, though I will say they can be a bit annoying.
Black toenails usually develop because as one runs, his foot can slide forward just a bit and bump up against the front of his shoe. This causes damage to the tissue under the toenail and capillaries then burst, leaving black blood underneath the nail. Once a nail turns black it begins the process of falling off, which can take quite some time, often several weeks or months.
In the time in between a nail turning black and falling off, a new nail usually begins forming underneath the black toenail (which slowly pushes the black nail out). In almost every situation, just waiting for the toenail to fall off is the best solution. Once it is ready to fall off, it will do so painlessly. If the toenail hurts, it is recommended that you contact a doctor.
I have seen articles that say you can poke a hole in the top of the black nail
with a hot paperclip to release the fluid, but that can lead to infection or
excessive pain. Personally, I have never met a single person who has ever had
to do this and I do not advise it.
There is one simple solution that helps prevent bruised toenails: properly fitting shoes. As stated earlier, the most common cause of a bruised toenail is that the foot slides forward in the shoe and bumps up against the front.
When purchasing a pair of shoes, runners should look for a pair that has a bit of room in the front of the toe-box of a shoe, though not too much room. If a shoe is too tight, the toe will constantly bump up against the front of the shoe. If the shoe is too roomy, the toe will slide forward very hard and bang against the front of a shoe. The perfect shoe will have just a little room in the toe-box (about a half an inch) and will be able to be tied securely to minimize slippage.
One final note to consider is that when the weather is hot or when a run is long, your feet will swell, so they will be longer at the end of runs than at the beginning. I recommend going to a local run-specialty store right after running to be fitted for the perfect shoe.
Try as one might, though, some people get black toenails more frequently than others even when wearing proper shoes. For those runners, and indeed all runners, I have a few suggestions.
First, accept that bruised toenails are a part of our sport. Second, remember that bruised toenails are painless and try to forget about them. Third, and this is those of you with a fancy event coming up, invest in a pair of close-toed shoes or copious amounts of nail polish because if you are a runner, you never know when a bruised toenail is going to pop up.