What's the cause of this painful problem? Chafing. When fabric rubs against the nipple, the nipple becomes irritated, then chafed, or worse. Many runners don't experience the dreaded chafed nipple until they begin doing longer runs. The longer the time the fabric has to rub against the nipple the higher the chances of irritation.
So, what's a runner to do? First thing, make sure your running tops are not made of cotton. Cotton absorbs and retains moisture which can increase the irritation. When wet, cotton also tends to be abrasive against the skin. Finding non-cotton running apparel shouldn't be too hard these days with all the technical fabrics available. Look for tops made of polyester, nylon, or spandex. Some trademarked technical fabrics include Lyrca, CoolMax, DriFit, FitDry, ClimaLite, and PlayDry. Be careful though, just because a top is made of a technical fabric doesn't mean you're safe—design and fit also plays a big part. Look out for irritating seams. Flatseam stitching will help minimize chafing. Also, decide if you're a tight-fit or a loose-fit. I've discovered that shirts or singlets that contain Lycra or spandex and are designed to have a body-hugging fit, cause less chafing. The fabric doesn't move around as much creating less friction and irritation. However, body-hugging fabrics aren't for everyone. Some runners prefer loose-fitting tops so that if the irritation begins, they can lift the fabric away from affected area.
You've tried various types of fabrics, loose-fit, snug-fit, and you're still getting runners' nipple? Then you might be in need of one of the many products designed to stop nipple chafing.
Lubricants: Bag Balm, Udder Cream, Body Glide , Sportslube, SportShield, SportSlick, are all products designed to lubricate and provide a protective barrier between the nipple and the fabric. Petroleum jelly is also a favorite. Be careful though, some of these products may leave stains. These products are also good to use on other areas of the body where chafing may occur.
Protective Coverings: NipGuards are popular with many runners. NipGuards adhere directly to the base of the nipple and are 100% guaranteed not to fall off until you decide to remove them. The premise is simple. You cover the nipple and they don't get chafed. Many runners also use regular adhesive bandages such as Nexcare or Band-Aid. If you decide to go this route, be sure to select one of the many waterproof varieties on the market. Once, I used circular waterproof bandages and they worked fine. So fine in fact, that when I removed my shirt, I had two miniature water balloons attached to my chest. All I needed were some tassels and a dancer's pole. :-)
If you do experience severe chafing, treat the area like any open wound. Even though it will be painful, you'll need to wash and clean the area to prevent infection and then cover with a breathable bandage. Applying an antibiotic cream or ointment may be helpful too.