It is a myth that the height can be increased beyond one's genetic pre decided growth pattern. It is under the control of growth hormone and one can grow as long as the epiphyseal fusion has not taken place in bones and that often occurs till 18 years of age. If you are already 21, then I don't want to give any false hopes, the chances of increasing height further is less or minimal. there are various powders available in the market which are steroid based, but they cause more harm and no benefit usually.
I hope I have answered your query,
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