Warts are small growths on the skin caused by a virus. They're unsightly, sometimes contagious, and above all, tricky to get rid of. Especially difficult to deal with are subungal and periungual warts, which appear under the fingernails and toenails. Because of their hard-to-reach placement, they require a special approach to eradication.
Treating warts under fingernails and toenails
Identify them. Subungal and periungal warts look similar to other warts: ugly, rough bumps with uneven borders that sometimes resemble cauliflower. They are usually lighter or darker than the skin around them.
File down nails and the wart. Warts under the nail are more difficult to get rid of than other warts because they are difficult to reach. One option is to make them easier to treat by clipping and filing nails away from the wart; this is only an option if the wart is closer to the tip of the nail than to the base. Soak your finger in warm water, then file away the edge of the nail and across the top of the nail with a smoother file to make the nail thinner. If you can, also file away at whatever part of the wart you can reach. Then be sure to throw away the file to avoid spreading more warts.
Try over-the-counter treatments. Apply over-the-counter treatments to your wart, following the directions on the packaging. After you've applied the treatment, cover with a bandage and repeat regularly, based on the product's directions.
See a doctor. If after trying home remedies, your warts still haven't cleared, or if you're unable to get to the wart because it's too far down on the nail bed, see your doctor. A general practitioner or a dermatologist can prescribe stronger medications and can also refer you to laser-based treatments, which can get at the wart without damaging the nail above.
Not all over-the-counter treatments are created equal. Look for salicylic acid in the highest concentration you can find.
Remember that most warts will go away on their own eventually, and that it's likely that warts, treated or not, might return.
Prevent further infection to yourself and to others by keeping your nails short (to discourage biting, which can open the route to infection), and by washing your hands on a regular basis.