Some kinds of black salve are apparently more "potent" than others. If you aren't careful, you can burn a hole nearly to the bone. The more you use, the bigger the sore. A big sore can take months and months to heal, turn green and mildew. They say to clean it twice a day with hydrogen peroxide and to keep vasoline or vitamin E oil around the edges, but I was cleaning mine 4-5 times per day. Vasoline definitely worked a lot better than Vitamin E oil. I believe it is the zinc chloride that creates the terrible burn. I would never, ever recommend taking anything with zinc chloride internally. Bloodroot yes, zinc choride no.