Final answer:
Aripiprazole is the antipsychotic that has the least effect on prolactin levels due to its unique function as a dopamine D2 receptor partial agonist, which differs from risperidone and other antipsychotics that increase prolactin through D2 receptor antagonism.
Step-by-step explanation:
The antipsychotic medication with the least effect on prolactin is generally considered to be aripiprazole. Unlike other antipsychotics that tend to increase prolactin levels due to dopamine D2 receptor antagonism, aripiprazole is a dopamine D2 receptor partial agonist. This unique mechanism of action means aripiprazole can modulate dopamine activity without causing the significant increase in prolactin seen with other antipsychotic drugs. In the case of risperidone (Risperdal), while effective in treating symptoms of psychosis such as hallucinations, delusions, and incoherence, it may increase prolactin levels because it strongly antagonizes D2 receptors as well as serotonin type 2 and other receptors.