Your condition most probably has been diagnosed by your treating doctor as Vitiligo
hence prescribed Tacrolimus and melgain local applications.
It takes about three months of regular applications before you start to see any improvement in the colour of white patch
Tacrolimus is a drug known to be Immune modulatar and is effective in Vitiligo as one of the theory of cause of vitiligo is considered to be abnormal immune response of body towards pigment producing cells.It is better than steroids applications as iot does not thin out the affected skin and less chances of harmful systemic effects of long term use of steroids.
However porlonged use of tacrolimus is not recommended as rare cases of skin cancer
have been reported.
Consultant Cosmetic and Plastic Surgery