A 2006 double-blind, randomized controlled trial indicated ondansetron may have value in the treatment of schizophrenia, as an adjunct to haloperidol. The study found the combination to significantly improve negative schizophrenia symptoms, and people taking both drugs experienced fewer of the adverse effects commonly associated with haloperidol.[28] An earlier, smaller, open-label trial had found ondansetron to be useful in treating antipsychotic-induced tardive dyskinesia in people with schizophrenia, and the study patients also showed significant improvement in the disease’s symptoms.[29][30]
Early studies have also examined ondansetron as a possible treatment for psychosis resulting from advanced Parkinson’s disease.[31] Its apparent benefits despite a lack of any significant antagonistic properties at dopamine receptors or the 5-HT2A receptor raises interesting questions about the etiology of psychosis.
Antipsychotics reduce positive symptoms of schizophrenia (delusions, hallucinations and disordered thought), but have undesirable side effects. Moreover, satisfactory treatment of negative symptoms (apathy, poverty of speech, lack of interest in social interactions) and cognitive dysfunction is currently not available. The selective 5-HT2CR full agonist vabicaserin showed antipsychotic efficacy with fewer side effects than olanzapine. Adjunctive pimavanserin (a selective 5-HT2AR inverse agonist) facilitated antipsychotic dose and side-effect reductions. Selective 5-HT3R antagonists (ondansetron, tropisetron and granisetron) showed positive results on negative symptoms and/or cognitive impairments in phase II trials. Adjunctive ondansetron has now entered a phase III trial for such indications. Finally, 5-HTA5R and 5-HT7R antagonists have shown procognitive actions in animal models of schizophrenia. These novel serotonergic drugs seem promising for improving the current treatment of schizophrenia.