Background:Global health agencies agree that infants should not be fed complementary foods before 4 to 6 months of age. However, given the World Health Organization (WHO) definition of complementary food as "anything other than breast milk," little is known about the relative risks of formula compared with other complementary foods on infant health. This article aims to fill this gap in the literature, by assessing how differences in the timing of the introduction of nonformula complementary food between breastfed and formula-fed infants impacts infant health.Methods:Eight health outcomes by complementary food introduction, breast-feeding, formula feeding, and mixed feeding (breastfed and formula-fed) were predicted using logistic regression with generalized estimating equations on the newborn through 6-month waves of the Infant Feeding Practices Study II (IFPS-II).Results:Complementary foods increased the likelihood for all health risks measured. Given greater prevalence of early complementary food introduction among formula-fed infants, most health differences between breast-feeding groups shift to nonsignificance in full models, with the exception of higher rates of hard stool and cough/wheeze among formula-fed and mixed-fed infants but lower rates of diarrhea (LO = -0.577; 95% confidence interval [CI] = -1.074 to 0.080) and runny nose or cold (LO = -3.19; 95% CI = -0.552 to -0.086) for mixed-fed than breastfed infants.Conclusions:Our results confirm health benefits of exclusive breast-feeding and that the introduction of complementary foods before 4 to 6 months poses a greater risk to infant health than does formula. Greater attention to the early introduction of complementary foods is needed in research and clinical practice.