Phosphodiesterase 9 inhibitors or PDE9 inhibitors are a class of drugs that work by inhibiting the activity of PDE9. The first compound with this effect, BAY 73-6691, was reported in 2004. [1] PDE9 inhibitors are under investigation for the treatment of obesity, [2] hepatic fibrosis, [3] Alzheimer's disease, schizophrenia, other psychotic disorders, heart failure, and sickle cell anemia. [1] Drug candidates include CRD-733, [4] osoresnontrine, tovinontrine, and PF-04447943. [1] Cannabidiol acts as a PDE9 inhibitor in vitro. [5] There are no PDE9 inhibitors that have been approved as of 2023. [1]