Don't do it!!!
10 mg is the approved dose in the USA, Although the adult dose of Loratadine
in other parts of the world is 20 mg.
Taking higher doses, will increase your chances of developing heart rhythm problems. High doses of Loratadine have increased the QT interval of some patients. See: http://www.nps.org.au/publications/health-professional/health-news-evidence/2013/drugs-QT-invterval