The CDE excipient database pointed out that leucine is used as a drug carrier in pharmaceuticals to enhance the solubility of the drug or enhance the effect of the drug, and reduce side effects. Leucine is an essential amino acid for the human body, a nutrient ingredient, non-toxic, and generally considered safe. The FDA IIG excipient database has a single-dose maximum dosage of 526 mg for intravenous infusion of leucine, and a maximum single-dose oral dosage of 4.5 mg, but no data on the safe dosage of inhaled preparations has been found. Leucine has outstanding performance in improving the powder properties and aerodynamic properties of dry powder formulations. It provides an excellent solution for the preparation of dry powder formulations. Although leucine is a small molecule metabolite necessary for the body, researchers still face the problem of the safe dosage of leucine for inhalation. Therefore, how or how to use leucine in inhalation preparations and to explore its safe dosage is also an arduous challenge.