Dark Underarms can be caused by 1) Reaction to Deodarants or perfumes 2) Shaving or waxing 3) Friction from tight clothes 4) Acanthosis : This is genetic condition where skin folds tend to become dark 5) Infection : Some fungal or bacterial infection can cause under arm pigmentation
Next Steps
The best treatment for under arms pigmentation is Q switch Laser which removes the pigmentation effectively and is completely safe. Also we need to find out the root cause of under arm pigmentation and treat that as well.
Health Tips
This combined approach will solve your problem permanently. All the best. Regards, Dr Jeetendra Khatuja ( Mumbai)