Being underweight can be associated with multiple factors. So, it's not possible for me to answer that question on an individual basis without having lots more info.
Generally speaking, however, although you may lose a little more weight during the process, cleansing is the best way to gain weight in the long run. There is lots of info on cleansing on this site that can help you with that endeavor if you haven't started already.
Also, I think it's important these days to really look at whether or not you are *really* underweight or does it just appear that way because so many people are overweight and that has become the norm.
How you feel and your overall health are most important - not how much you weigh. We've become kind of obsessed with weight in this country.
Hope that helps. Happy New Year! - Kelly