SYMPTOMS: A nail, usually on the big toe, looks dark and blackened, and may fall off.
WHY IT HAPPENS: "Black nails occur as a result of microtrauma," says runner Dr. Robert Klein, a podiatric physician and foot surgeon. "As you run, the top of the shoe rubs against the toe." The impact can cause blood to collect under the nail; if it isn't aspirated, the extra blood and fluid can eventually separate the nail from its bed, causing the nail to fall off.
As the weather warms up, you may be more prone to black toenails because your feet swell, making your shoes tighter and the friction greater. Runners who regularly do hill workouts are at higher risk because the feet hit the front of the shoes repeatedly on the downhill efforts.
HOW TO DEAL: Don't force the nail off; let it detach or have your doctor remove it. Then put a bandage around the toe to cushion the nail bed from trauma, Klein recommends. Once the nail separates, a thin, new nail grows underneath it that must stay protected. If you notice redness and swelling after more than a week or two, see your doctor immediately.
To prevent this condition, keep your toenails trimmed and wear wicking socks to help your feet stay dry and to prevent slipping inside your shoes.
To hide an unsightly black nail, it's OK to polish the damaged nail and/or the new nail that has grown underneath it. Before you apply a color, try Nailtiques Nail Protein Formula 2 ($10, nailtiques.com), which uses keratin, protein, gelatin and calcium to build a healthy nail foundation.
SYMPTOMS: A fluid-filled bubble develops where your shoe has been rubbing against your skin.
WHY IT HAPPENS: "Blisters are extremely common with new runners," Klein says. "The skin might not have toughened up yet, or you might not be wearing the right socks."
Even if you're a longtime runner, poorly fitting shoes can cause chafing. "Many women sacrifice the condition of their feet by wearing shoes that are a half-size too big or too small," says Dr. Diane Berson, dermatologist and skincare consultant. "The side effects of this include blisters and cracked heels as shoes either rub against the feet all day or pinch too tightly."
HOW TO DEAL: Break in your sneakers before you attempt long-distance running. Klein recommends wearing technical socks of a nylon/Lycra blend to wick away moisture and to support the foot. He also suggests applying FootGlide Anti-Blister Foot Formula ($9, drugstores), a roll-on balm, "anywhere you're prone to chafing." For a long run, try Nexcare Active Waterproof Blister Pads ($5, shop3m.com) on your trouble spots.