A rash in the armpit fold can be caused by various reasons, such as contact with an irritating product or eczema. In general, rashes can often go away on their own. You can try applying a fragrance-free cream with colloidal oatmeal and warm compresses to the area. Keeping the area dry and clean, changing your shirt if necessary, can also help. If the rash is caused by a fungus, an antifungal cream like clotrimazole 1% may be needed. For eczema or rash without a fungal component, a hydrocortisone cream may be recommended. Additional medication treatment will depend on the cause and level of discomfort experienced.