A medical device including an array of microneedles and a coating disposed on or within the microneedles and a method of making such a device are disclosed. The coating includes a peptide therapeutic agent and an amino acid. A method of stabilizing a peptide therapeutic agent with an amino acid on an array of microneedles is also disclosed. In some cases, the peptide therapeutic agent and the amino acid either both have a net positive charge or both have a net negative charge. In some cases, the peptide therapeutic agent is histidine.