You can use a hair concealer for the 4-5 months after surgery, especially until the transplanted hair starts growing. One thing to keep in mind is the wash your hair before going to bed or in the evening when you do not need the effects of the concealer anymore. This is for protecting the tissues, the pores from blockage. Otherwise, you can use a hair concealer with no problem. It will have no side effects if used after a hair transplant.