How Do You Whiten Dark Underarms Due to Deodorant Burn?

I would recommend that you see a board-certified dermatologist before you seek treatment. A condition called acanthosis nigricans commonly presents as darkening of the armpit skin. Acanthosis nigricans may be genetic, or associated with obesity, diabetes or other thyroid disease or even cancer. A dermatologist will make a dagnosis and suggest appropriate treatment.

This can be a difficult problem to treat. The best approach would be to see a dermatologist to determine true cause ie genetic predisposition versus external contact with post inflammatory hyperpigmentation. There are bleaching creams that may be tried under physician supervision and by prescription.