Dark Under arms can be caused by multiple factors, most common being waxing or shaving, deodorants, acanthosis, hormonal changes,etc.
First step is always correct diagnosis to find the reason for your pigmentation. Also we may need to do some blood tests to check for
Vitamin deficiencies,etc.
Next Steps
Once we have figured out the reason, we can do various treatments like Microdermabrasion, Peels, Laser,etc to lighten the dark under arms. Laser being the most effective treatment option.
Health Tips
I would suggest you consult once so that we can check properly and recommend the best treatment for you. All the best. Regards, Dr.Jeetendra Khatuja ( Mumbai)