Dark underarms are a very common complaint from a lot of women, and the problem is usually an extremely minor one. Unless the skin itself is permanently discolored, there is no need for any real treatment, including laser treatment and other cosmetic surgery procedures. The underarms appear dark for two reasons — friction and hair under the skin.
The underarms usually experience a lot of natural friction, in addition to which we subject them to further friction by wearing tight clothes. This friction is what darkens the skin over a period of time. You should therefore avoid wearing clothing that is too tight, particularly at the underarms. Occasionally wearing a tight fitting top is okay (even then, it should be a comfortable fit), but doing so on a regular basis is likely to darken your underarm skin.
In addition, from puberty onwards, our underarms develop rather thick hair, and most women start shaving their underarm hair soon after it appears — this is why your underarms have been dark since your teens. Shaving only removes part of the hair, leaving the root and a little bit of the hair shaft under the skin, and making the skin appear dark. If you feel it is worth the pain, you should instead start waxing your underarms — this removes the entire hair, root and all.
answered by M W