Yes, it is highly recommended to use washing detergent.

Milk contains fat and protein that cannot be washed away using just plain water.  Before sterilizing the bottles, be sure to wash them clean with washing detergent and rinse them thoroughly.

We recommend using Pigeon liquid cleanser to wash the bottles and nipples as it is made with 100% food grade ingredients, and therefore a safer option as compared with general dish-washing detergent.