Most people don't spend a lot of time closely examining their own armpits, so discovering a pimple-like bump under your arm in the shower can be unnerving. Unlike hard or rubbery lumps, which you should have checked out by a dermatologist, pimple-like bumps are usually benign and fairly easy to treat.
Pimple-like bumps under your arms are usually caused by one of two problems. Most commonly, they're caused by ingrown hairs, which occur when a hair's sharp point digs back into your skin, causing an irritated bump. If you shave under your arms, ingrown hairs are the most likely cause of bumps under your arms. If you don't shave your underarms and your bumps are hard like a marble, your bumps may be caused by sebaceous cysts, says Ginni Mansberg, a Sydney, Australia-based general practitioner.
If you're African American, Hispanic or have curly hair, you may be more likely to get ingrown hairs under your arm and on other parts of your body. That's because a curved hair follicle -- the kind that makes hair curl -- is more likely to twist back on itself and penetrate your skin.
The most effective way to prevent bumps caused by ingrown hairs is to stop shaving under your arms, but if that's not an option, you can take other measures to reduce your risk. Use a moisturizing shave cream and a sharp new razor, and press lightly on your underarms so that you're less likely to push bacteria back into the skin. To reduce your risk for cysts, exfoliate regularly and stick with oil-free and non-comedogenic body creams and deodorants.
If your bumps are caused by ingrown hairs, scrub your underarms gently with a rough, wet cloth once or twice a day to help exfoliate skin cells holding the trapped hair in place. Treat irritation by dabbing a little cortisone cream directly onto the bumps that are bothering you, recommends Debra Wattenberg, MD, assistant professor of dermatology at Mount Sinai Hospital in New York City. There's no real treatment that works for cysts, if they're causing your bumps -- most people just have to learn to cope with them unless they want to have them removed by a dermatologist, says Mansberg.
Because your underarms don't get a lot of sun exposure, skin cancer is rare there -- but not impossible. If the bumps under your arms are sore, if they bleed, itch or don't heal for a long time, see a dermatologist to make sure they're not caused by basal cell carcinoma, a common and treatable type of skin cancer.