A boil-drawing salve is an ointment that reduces inflammation. Applying the salve kills bacteria or other agents that cause infection.
Generally, the staphylococcus aureus bacteria causes boils. Staph infects hair follicles or breaks in the skin, and inflammation results. Ichthammol is a black ointment containing sulfonated shale oil. It is a convenient and common over-the-counter remedy found at most drug stores.
Prescription antibiotics like Mupirocin are another remedy for boils. Milk thistle, a natural remedy, is an ancient solution for boils. The herbal remedy goes back nearly 2000 years. Users soak gauze in milk thistle, then apply it to boils.