It is quite ok for a preterm baby to receive formula milk and is not dangerous if the correct formula is given. Preterm babies will be given special formula milk that is produced specifically for them 1. These formulas are very specialised and prescribed by a Doctor to meet the individual nutritional requirements of each preterm baby as they grow.
All artificial milks or modified infant formula are highly processed products and have gone through rigorous health and safety checks. Under UK Law it is an offence to sell any infant formula as being suitable from birth unless it conforms to the compositional and other criteria set out in the Infant Formula and Follow-on Formula Regulations 1995 2.
1Kelnar C, Harvey D. (1987). The sick newborn baby. Bailliere Tindall.
2HMSO 1995. Food. The infant formula and follow-on formula regulations.