Zinc is an important element that can result in a delay in wound healing if there is a deficiency. Zinc supplements have also been advocated to treat verruca or warts.
A corn plaster is a medicated pad that is supposed to each away at a corn. The acid that is used can not tell what is corn and what is normal skin, so it just eats away at whatever it is put on. That can be dangerous for those who are at sick (eg diabetes).