Never use this to treat skin cancer:

http://www.dailymail.co.uk/health/article-2600537/Black-salve-ointment-leaves-man-inch-wide-HOLE-head.html

Man left with an inch-wide HOLE in his head after using ‘quack’ ointment to treat skin cancer

Australian, 55, had inch-wide hole in his temple after using ‘drawing salve’
Man used Black salve on side of his head for four months
Black salve is […]