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Because it's better absorbed transdermally than orally.
https://books.google.com/books?hl=en&lr=&id=p0KriJuFl1IC&oi=fnd&pg=PP2&ots=FN8le0EkhP&sig=6Anpd6oj4bTSGI0r8n1kdiQnZeY#v=onepage&q&f=false
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