Can Formula Milk Be Fed To Newborn?

Hi, my wife has just given birth to a baby. She is not generating enough breast milk. When I checked with my paediatrician, he told me that we should continue with breast milk and do not try formula milk at all as it causes diarrhoea. We did the same bit due to lack of feed baby loses 400 gm weight. We got worried and started nan pro 1. The baby is now 6 days old and regained weight. But I am worried about the long term impact of formula milk. Please help