What Causes Re-occurrence Of White Patch Under The Lower Lips?

hello doctor..my name is pravallika.i am 19yrs old..since 1yr i have small white patch on my lower lip.it is very small.i have consulted the doctor..he recommended oxylen for 2months.after 2months he recommended melanocyl.i am not using it regularly because i have many doubts.most of the time that white patch is not visible..please give me suggestions to cure that small patch..