Hi, It would have been better if you have mentioned your age and sex (male/female). Armpit is a hairy area with lymph nodes which drain from the breast and anterior chest wall. The painful boil can be a inflamed hair follicle (folliculities), which is the most common cause. It can also be a lymph node inflammation and enlargement. Folliculities will have severe pain and redness around the area of boil. Fucidin cream is appropriate for that. In addition you can take hot fomentation over the swelling. If the pain continues, you have to take a course of oral antibiotics. If the swelling is not getting subsided after antibiotic therapy then you have to get a clinical examination of breast and anterior chest wall being done from a physician. Hope I have answered your query. Let me know if I can assist you further. Regards, Dr. Iven Romic Rommstein, General Surgeon