Make sure that you do this very superficially, because if you draw blood, infection will be driven deeper into the tissue. Be sure to open the infected site to the air, and keep it very clean and dry. Systemically, there is no medicine that will cure canker if superficial debridement is not performed. After applying the topical medication, apply a clean, dry bandage that is waterproof. Canker prefers moist conditions, so you must keep the affected area dry.
Horses have variable response to treatment.
Some cases heal within a week or 10 days, and some cases last for months. Years ago, owners lost some horses to canker. That now is the exception rather than the rule because veterinarians are able to recognize and treat the disease early in its process. Given good, aggressive treatment, a week to 10 days of intensive therapy should have the canker under control. Once your horse is completely rid of canker, the condition is gone.