Background: Oral pediatric liquid medications are commonly used in children who suffer from chronic diseases. Due to acid components in their formulations, these medications may possess a high erosive potential to dental tissues. Erosion is one of the major causes of degradation and demineralization of the enamel. If demineralization causes are not inhibited, it progresses and completely degrades the enamel. So, the dentin is exposed, and, finally, the pulp will be engaged in the process. Aim: This study aimed to evaluate the remineralizing properties of casein phosphopeptide-amorphous calcium phosphate (CPP-ACP) and nanohydroxyapatite on erosive lesions caused by acetaminophen syrup intake in vitro. Methods: The microhardness of 34 healthy human primary molars was determined prior to and following a seven-day, three-times-a-day, one-minute exposure to acetaminophen syrup. The teeth were then randomly divided into four groups and were immersed in artificial saliva, fluoride gel, nano-hydroxyapatite, and CPP-ACP for 10 minutes. Then, their final microhardness was measured, and repeated-measures ANOVA analyzed the data. Results: Enamel microhardness in two groups decreased after exposure to acetaminophen syrup and increased after applying remineralization agents, CPP-ACP, and nano-hydroxyapatite. The two groups did not show significant differences in terms of microhardness increase (p-value=0.141). Conclusions: According to the findings of this study, CPP-ACP and nano-hydroxyapatite can improve the microhardness of enamel, which has been reduced as a result of exposure to medicinal syrups.