soap for hair

Hello, my question is: can i use a piece of ordinary toilet soap instead of a shampoo? Some say it’s really harmful, others say smth like “my grandma has been using it all her life and still has wonderful hair”. When i tried it i loved the volume it gave my blond thin hair (after soap i also used a conditioner), so i’d love to use it all the time, but afraid that it can make my hair dry and fragile. Thanks in advance:)