Selective serotonin inverse agonists (SSIAs) are used to treat hallucinations and delusions from Parkinson's disease psychosis. It's not clear how they work, but it may be by targeting serotonin receptors in the brain, which helps to balance mood and reduce symptoms like hallucinations and delusions. These medications are typically available in oral forms. Pimavanserin (Nuplazid) is an example of an SSIA.