The effect appears to be strong in those up to 12 months old, but weaker in older infants than newborns, Denise Harrison of Royal Children's Hospital in Melbourne and colleagues reported online in Archives of Disease in Childhood.
Still, sucrose or glucose should be considered before and during injections, they said.
"Based on extensive evidence of the efficacy of sweet solutions in neonates and the evidence from this systematic review, sucrose or glucose along with other recommended physical or psychological pain reduction strategies, such as non-nutritive sucking, breast-feeding, or effective means of distraction, should be consistently utilized for immunization," they wrote.
"This information is important for healthcare professionals working with infants in both inpatient and outpatient settings, as sweet solutions are readily available, have a very short onset of time to analgesia, are inexpensive, and are easy to administer."
The efficacy of sugar solutions in reducing pain from minor procedures has been well-established for newborns, but the evidence wasn't as clear for infants older than one month.
To evaluate the evidence, Harrison and her colleagues performed a systematic review of 14 randomized controlled trials that included 1,674 total injections. Oral glucose or sucrose solutions were compared with water or no treatment.
Ten of the studies used sucrose solutions in concentrations ranging from 12 percent to 75 percent. A 30 percent glucose solution was used in another three studies. A final study evaluated two sucrose concentrations -- 25 percent and 50 percent -- and a 40 percent glucose solution compared with water.
Volumes were generally 2 mL or less, although one study used 10 mL of 25 percent sucrose.
In 13 of the 14 studies, sugar solutions of various concentrations administered during or after an immunization decreased crying incidence and duration, as well as composite pain scores. In the 14th study, a 12 percent sucrose solution was no better than sterile water.
Neither sucrose nor glucose significantly reduced mean heart rate, change in heart rate from baseline, or heart rate variability.
The researchers also looked at three different aspects of the crying to determine how well this sweet approach worked. These aspectes were proportion, duration, and incidence.
In three studies, 50 percent sucrose or 40 percent glucose resulted in an average 10 percent reduction in the proportion of crying time compared with placebo.
In six studies, all but one using sucrose, there was a nonsignificant reduction of 16 seconds in crying duration with the sugar solutions. However, when two studies using 12 percent sucrose were excluded, the reduction became statistically significant, although the difference was clinically small, according to the researchers.
Finally, in three studies evaluating a 30 percent glucose solution, there was a 20 percent relative risk reduction in crying incidence. Considering an absolute risk reduction of about 17 percent, the number needed to treat was six.
The optimal dose of sucrose or glucose could not be determined because of between-study variations in volumes and concentrations, according to Harrison and her colleagues.
"Although sucrose is the sweetest of the sugars ... either sucrose or glucose could be used depending on availability and organizational preference as long as the solutions are sufficiently sweet (i.e., at least 30 percent glucose or 24 percent sucrose)," they wrote.
The authors acknowledged some limitations of the review, including the fact that variation across studies in the concentration of the sugar solutions, outcome measures, and timing of outcome assessment precluded inclusion of most of the studies in the meta-analyses.
Also, the researchers were not able to evaluate varying pain responses according to type of immunization, order of shots, or injection techniques.