DIY enthusiasts and professionals alike need to be fully aware that using the correct backing material for ceramic tiles in bathrooms is absolutely necessary. Regular gypsum board or drywall is completely unacceptable as a backing board for tiles in a wet environment.
Suitable materials must be waterproof and include cement backer board or fibreglass reinforced silicone-treated gypsum board product. The backing board should be fastened to the studs with hot-dipped galvanized nails.
Source: Chillicothe Gazette