Antipsychotics block dopamine (and in second-generation drugs, serotonin) receptors to reduce positive psychotic symptoms. First-generation drugs have higher motor side effects; second-generation drugs have metabolic side effects but better tolerability.
From your study of schizophrenia spectrum disorders, you know the distinction between positive symptoms (hallucinations, delusions, disorganized thinking — symptoms that are additions to normal experience) and negative symptoms (flat affect, alogia, avolition — subtractions from normal experience). Antipsychotic medications were discovered accidentally in the 1950s, when chlorpromazine, developed as a surgical anesthetic adjunct, was found to dramatically calm psychotic agitation. The key observation was that these drugs produced a specific kind of quieting — not sedation, but a blunting of the delusional preoccupation and hallucinatory salience that drives psychotic distress.
The mechanism centers on the dopamine D2 receptor. The mesolimbic dopamine pathway — from the ventral tegmental area to the nucleus accumbens and limbic structures — is hyperactive in psychosis, generating the aberrant salience that makes random stimuli feel loaded with threatening significance. All effective antipsychotics block D2 receptors in this pathway, reducing that pathological signal. The dose required to achieve clinical antipsychotic effect corresponds closely to the degree of D2 blockade — a tight structure-function relationship that is unusual in psychiatry. First-generation (typical) antipsychotics like haloperidol block D2 receptors throughout the brain without selectivity, which also affects the nigrostriatal pathway (controlling motor function) and produces the characteristic extrapyramidal side effects: akathisia (inner restlessness and compulsion to move), Parkinsonism (tremor, rigidity, bradykinesia), and with long-term use, tardive dyskinesia — involuntary repetitive movements, often of the mouth and face, that can persist after the drug is stopped.
Second-generation (atypical) antipsychotics like clozapine, olanzapine, and risperidone add serotonin 5-HT2A receptor blockade to D2 blockade. The serotonergic component modulates dopamine release in a pathway-specific way, partially restoring function in the mesocortical pathway (thought to underlie negative symptoms and cognitive deficits) while still reducing mesolimbic hyperactivity. The result is lower rates of extrapyramidal side effects and, in some cases, modest improvement in negative symptoms and cognition. The tradeoff is metabolic syndrome: weight gain, elevated triglycerides, glucose dysregulation, and increased diabetes risk — particularly severe with clozapine and olanzapine. Clozapine, despite being the most effective antipsychotic for treatment-resistant cases, carries a rare but serious risk of agranulocytosis (white blood cell destruction) requiring mandatory weekly blood monitoring.
The clinical picture is one of imperfect tools that have transformed the treatment of psychosis without solving it. Antipsychotics are highly effective at suppressing positive symptoms — the florid hallucinations and delusions that make acute psychosis so distressing and dangerous — but do little for negative symptoms and cognitive impairment, which are often the bigger determinants of long-term functional outcomes. Medication adherence is a profound clinical challenge: the side effect burden is real and experienced subjectively (weight gain is visible, akathisia is miserable, sexual side effects are common), and many patients lack insight into their own psychosis, making the cost-benefit calculation feel different from their perspective than from their clinician's. Long-acting injectable formulations exist precisely to address the adherence problem. Understanding antipsychotics requires holding both their remarkable effectiveness in reducing psychotic suffering *and* their significant limitations in restoring full function and quality of life.