Your friend is ABSOLUTELY WRONG! Formula milk is made from cow's milk - I assume you are producing human's milk!
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Seriously, though, breastmilk is SO much better for babies, it changes and develops according to your baby's needs. It contains antibodies for viruses, infections etc that you have been in contact with, which means if there's a cold going round and you make antibodies you will give those to your baby, giving him/her some protection. Evidence proves that if you feed a baby with formula instead of the way nature intended you increase their risk of gastroenteritis, infections, asthma, eczema, allergies, and possibly even diabetes and obesity later in life. Obviously, there are no gurantees, but there is no guarantee that if you don't smoke you won't get heart disease or lung cancer, but it would be ridiculous to say that makes not smoking pointless!!
People unfortunately are not used to babies being breastfed - most women having babies these days were probably bottlefed and many have never seen a baby being breastfed, so they think 'it never did me any harm'. True, most formula fed babies will be fairly healthy, but if you want to breastfeed your baby and give him/her the best possible start then don't let other people's negativity put you off!