You don't need formula for your baby to gain the appropriate weight. If your baby is not gaining weight appropriately with exclusive breastfeeding, then you might need to talk to your Doctor. Breast milk has all the nutrients your baby needs for the first 6 months including water.

Please don't ,you have to eat good and quality food and drinks a lot of water to helps in feeding the baby well.

Please exclusively breastfeed. Breastmilk contains every thing your baby needs to be healthy.