The treatment doesn't hurt when the product is used correctly and in accordance with the user instructions. However, formic acid in itself is corrosive and needs to be handled with extra care. It is very important not to let the solution come in contact with the skin surrounding the wart - as it can damage the skin.
If there is any burning or stinging sensation while treating, rinse the wart/verruca with cold water and do not apply any more solution. It is also very important to never cover the wart with plasters during treatment with EndWarts, the effect will be too strong and can cause skin damage.