Melasma is usually bilateral. However atypical things do occur. Exact aetiology is not known. Circumstantial evidence suggests that it has to do with the alterations of hormones. These sensitize the skin to UV rays, as a result of which hyper-pigmentation occurs. Someday, hormonal imbalance will correct itself. Till then, one should protect from exposure to light, to the maximum extent possible. The hyper-pigmentation will reduce automatically. This is aided by the use of medicines, peeling or laser resurfacing. Therefore, you can deduce for yourself, that these are only supplemental in nature. However there is one caution. In a good number of Asians, significant post inflammatory pigmentation can occur which may not be acceptable.