Understanding armpit boils Boils develop when hair follicles or sweat glands become infected, leading to painful, pus-filled bumps beneath the skin.

5 harmful deodorant ingredients linked to painful armpit boils. The aluminum compounds, parabens, and other chemicals may be causing your underarm discomfort.

Causes of pain under the left armpit can range from minor skin infections to nerve damage. Learn about these and other causes, and when to see a doctor.