Vasoactive intestinal peptide is a twenty eight amino acid signalling molecule found in the gut, lungs and nervous system, involved in relaxing smooth muscle, regulating secretions and modulating inflammation. As a synthetic drug called aviptadil it was tested seriously during the pandemic for COVID-19 respiratory failure, including in a large trial run by the US National Institutes of Health that reported in 2023. That trial was negative, and the compound has not been approved. The nasal formulation popular in mould illness and chronic inflammatory response communities is a separate proposition again, used off-label with no approval and without the trial evidence the intravenous version at least accumulated.