Premmies are given sucrose before painful procedures because this seems to reduce the stress and discomfort of the babies. A recent study assessed the amount of crying following immunisations of babies up to 12 months of age. There were a total of 1674 injections. When babies were given sucrose or glucose before the needles, there was a modest reduction in crying and time crying. There seemed to be a dose-related effect in the older babies (the more sugar, the less the baby cried).
Immunisations are stressful for babies and parents and it may be helpful to make the experience less uncomfortable for all involved.
Ref: Harrison, D., Stevens, B., Bueno, M et al., Efficacy of sweet solutions for analgesia in infants between 1 and 12 months of age: a systematic review. Arch Dis Child 2010;95;406-13.