 [ Symptoms ] my adult son has an inflamed rash under his armpits that has blistered it even hurts to go in the shower what is it  has had it about a week [ Disease ] i am a paramedic and have dealt with something like that it turned out that the patient had changed deodorant and was allergic to a chemical in it if your son has swapped deodorants it is likely the case so have him stop use and take some benedryl for a couple days and should clear up