Objective To systematically investigate the impact of Dangmu extract syrup on the growth and development of 4-day-old (PND4) Sprague-Dawley (SD) rats and its toxicological reactions. Methods PND2 pups were randomly divided into a negative control group and low, medium, and high dose groups according to the whole litter design. Starting from PND4, the animals were orally administered with pure water or Dangmu extract syrup once daily for 18 consecutive days, followed by a 15-day recovery observation period after cessation of medication. During this period, the general condition, growth and development, neurological reflex function, spontaneous behavioral activity, hematology, coagulation, blood biochemistry, immune function, growth hormone, and histopathology of the animals in each dose group were assessed. Results At the end of the medication period (PND22), spleen weight and visceral system number of animals in all dose groups were increased compared with negative control group. Histopathological examination showed that spleen nodules did not form in the negative control group, and spleen nodules of animals in all dose groups were formed, larger in volume and more in number than those in the control group, and the degree of change was dose-dependent. After 15 days of recovery after administration, spleen weight of animals in all groups increased with growth and development compared with animals in the same group after administration, and the histopathological examination showed that the spleen structure of animals in all groups was more complete than that after administration, and the spleen nodule structure was obvious, but there was no significant difference between groups. The above changes were related to the development of animal organ structure accelerated by the nutrients contained in the syrup, and had no toxicological significance. No significant abnormal changes were found in other test results. Conclusion The splenic structure of 4-day-old SD rats can be improved in advance with no obvious toxic target organs. In clinical study, attention should be paid to controlling the dosage of the syrups, and regular monitoring of spleen and related blood and blood biochemical indexes should be carried out.