While its specific mechanism of action is unknown, it is believed paliperidone and risperidone act via similar, if not identical, pathways; therapeutic effect may be due to a combination of D2 and 5-HT2A receptor antagonism. Paliperidone also has antagonist effect at ±1 and ±2 adrenergic receptors and at H1 histamine receptors.
Paliperidone (as Invega) was approved by the FDA for the treatment of schizophrenia on December 20, 2006. This agent will initially be marketed for the treatment of schizophrenia and then for bipolar mania. Clinical trials of paliperidone for the treatment of schizoaffective disorder are also planned. It may also be used off-label for other conditions. The drug significantly reduces side-effects present in other anti-psychotic drugs formerly used to treat schizophrenia and bipolar disorder. Like risperidone, its possible use in people with autism and Asperger syndrome may be studied. Recently, the long-acting injectable form of paliperidone, marketed as INVEGA Sustenna, was approved by the FDA on July 31, 2009. 
Side Effects and Adverse reactions
According to the Invega website, the most common side effects of Invega are reported to be restlessness and extrapyramidal disorder, including involuntary movements, tremors and muscle stiffness. Invega website's Important Safety Information page includes this warning in bold type:
IMPORTANT SAFETY INFORMATION FOR INVEGA: Elderly Patients with dementia-related psychosis treated with atypical antipsychotic drugs are at an increased risk of death compared to placebo. INVEGA (paliperidone) is not approved for the treatment of patients with dementia-related psychosis.
The web page goes on to mention Neuroleptic Malignant Syndrome as a rare, but potentially lethal reaction to the medication. Heart rhythm changesâ€”potentially seriousâ€”may make Paliperidone risky for people with some heart conditions.
Other risks include:
- Tardive Dyskinesia, a serious, sometimes permanent side effect reported with INVEGA and other neuroleptics. The risk of tardive dyskinesia increases with total dose and thus becomes more likely the longer a person takes Invega.
- High blood sugar and diabetes
- Hyperprolactinemia, or elevated levels of the hormone prolactin, potentially leading to the absence of a menstrual period; breasts producing milk; the development of breasts by males; and erectile dysfunction.
- Complications associated with pre-existing narrowing or blockage of the gastrointestinal tract (esophagus, stomach or small or large intestine)
- Fainting or lightheadedness when standing up or sitting up too quickly.
- Impairment of alertness and driving ability
- Risk in people with a seizure disorder or a history or health conditions that make seizures more likely
- Extrapyramidal effects -- persistent movement disorders or muscle disturbances, such as restlessness, tremors, and muscle stiffness.
- Caution should be exercised before prescribing Invega to pregnant or nursing women.
- Increased sensitivity to heat, difficulty cooling off, and increased likelihood of dehydration.
- ^ "Prescribing Reference: New Product Releases - INVEGA". http://www.prescribingreference.com/products/showProducts/which/Invega. Retrieved 2008-05-25.
- ^ http://www.invega.com/invega/isi.html INVEGA Web site Important Safety Information page
- ^ http://www.invega.com/invega/isi.html INVEGA Web site Safety Information page