My underarm is getting darker from past 1 week and
Dermatitis, topical steroid, antihistaminic
Welcome to ask a doctor service.
I have gone through your query and here is my advice.
From your description, it seems to be dermatitis. You can upload a photo of the rash so that I am better able to help.
I would recommend you to apply topical steroid like clobetasol propionate cream on the affected area twice daily and take antihistaminic like tablet fexofenadine 180 mg twice daily for 10 days.
These are prescription drugs and should be taken under the supervision of the doctor.
Avoid applying soap on the affected area, don’t wear tight clothes and avoid perfumes and deodorants.
These will help in resolving your problem.
I hope to have answered your query and would be happy to answer any further queries.