Antipsychotic Medications —
First vs Second Generation

Haloperidol is the prototype high-potency first-generation antipsychotic and remains a clinical workhorse for acute psychosis and agitation. Its mechanism is almost purely dopamine D2 receptor blockade — with comparatively little activity at histamine, muscarinic, or alpha-adrenergic receptors. This narrow receptor profile means less sedation and fewer autonomic side effects than low-potency agents like chlorpromazine, but a substantially higher burden of extrapyramidal side effects.

EPS with haloperidol is a major clinical concern: akathisia (a distressing inner restlessness), acute dystonia (involuntary muscle contractions), and drug-induced parkinsonism (shuffling gait, rigidity, tremor) are all common, particularly early in treatment or at higher strengths. Long-term use carries the risk of tardive dyskinesia. Prophylactic anticholinergic drugs (such as benztropine) are often co-prescribed to mitigate these effects.

Haloperidol is available as oral tablets (available in 0.5mg, 1mg, 2mg, 5mg, 10mg, and 20mg tablets), oral concentrate, short-acting IM injection, and as Haldol Decanoate — a long-acting injectable formulation administered monthly, widely used to maintain adherence in patients with chronic psychotic disorders.

Chlorpromazine holds a singular place in psychiatric history: it was the first antipsychotic ever developed, introduced in France in 1952 and fundamentally transforming the treatment of psychosis. Before its discovery, there were essentially no effective pharmacological treatments for schizophrenia. Its development launched the entire field of psychopharmacology.

As a low-potency first-generation antipsychotic, chlorpromazine binds broadly across multiple receptor types beyond D2 — including histamine H1 (causing pronounced sedation and weight gain), muscarinic receptors (dry mouth, constipation, urinary retention), and alpha-1 adrenergic receptors (orthostatic hypotension, dizziness). This broad receptor activity makes it far more sedating and metabolically impactful than high-potency agents like haloperidol, but also means it produces fewer EPS at equivalent antipsychotic strengths.

Chlorpromazine is occasionally used off-label for intractable hiccups that do not respond to other treatments — a niche but well-established application. Available in 10mg, 25mg, 50mg, 100mg, and 200mg tablets, as well as suppositories and injectable forms.

Fluphenazine is a high-potency first-generation antipsychotic within the phenothiazine chemical class. Like haloperidol, it is primarily a D2 blocker with a narrow receptor binding profile — making it less sedating and less likely to cause autonomic side effects than low-potency agents, but carrying a high burden of extrapyramidal side effects.

Its primary clinical relevance today lies in its availability as a long-acting injectable (LAI) formulation — Fluphenazine Decanoate, administered every 2 to 4 weeks. This makes it a practical option for patients with adherence challenges, where the unpredictability of missed oral doses poses a significant relapse risk. LAI formulations in general are underutilized relative to their evidence base for preventing relapse.

Oral fluphenazine is available in 1mg, 2.5mg, 5mg, and 10mg tablets and a liquid concentrate. The decanoate LAI is administered intramuscularly by a healthcare provider. As with all high-potency typicals, the EPS risk — including tardive dyskinesia with long-term use — is a persistent clinical concern requiring regular monitoring.

Clozapine occupies a unique and paradoxical position in psychiatry: it is simultaneously the most effective antipsychotic ever developed and the one with the most significant safety monitoring requirements. For patients with schizophrenia who have failed two or more other antipsychotics, clozapine often achieves results that no other drug can.

Its receptor profile is broad and unusual — it binds dopamine D1, D2, D3, and D4 receptors, serotonin 5-HT2A and 5-HT2C receptors, histamine H1, muscarinic M1, and alpha-adrenergic receptors. This wide binding explains both its exceptional efficacy and its substantial side effect burden: pronounced sedation, significant weight gain, hypersalivation (paradoxically — a common and socially disruptive effect), constipation, and metabolic effects including elevated glucose and lipids.

EPS and tardive dyskinesia risk are very low with clozapine — a meaningful advantage for patients who have developed movement disorders on prior agents. It is available as 25mg, 50mg, 100mg, and 200mg tablets, and in an oral suspension form. There is no LAI formulation.

Olanzapine is one of the most effective and widely studied second-generation antipsychotics. Its efficacy for both positive and negative symptoms of schizophrenia is well established, and it is approved for both schizophrenia and bipolar disorder (as monotherapy and as an adjunct with lithium or valproate). In combination with fluoxetine (sold as Symbyax), it is FDA-approved for bipolar depression and treatment-resistant depression.

The defining clinical trade-off with olanzapine is metabolic. It carries the highest average weight gain of any atypical antipsychotic — clinical studies have documented average gains of approximately 7 kg (15 lb), and some patients gain substantially more. This weight gain is driven by potent histamine H1 and serotonin 5-HT2C blockade, which increase appetite, reduce satiety, and alter metabolic signaling. Olanzapine significantly raises the risk of metabolic syndrome, insulin resistance, dyslipidemia, and new-onset type 2 diabetes. Baseline and ongoing metabolic monitoring is essential.

EPS risk is low. Sedation is significant, which some patients experience as a benefit (particularly early in treatment), but which can impair function. Available in 2.5mg, 5mg, 7.5mg, 10mg, 15mg, and 20mg tablets; orally disintegrating tablets (Zydis); and a short-acting IM formulation for acute agitation. An extended-release injectable suspension (Zyprexa Relprevv) exists but carries its own monitoring requirements due to post-injection delirium/sedation syndrome.

Risperidone is the most extensively studied atypical antipsychotic and was among the first to be widely adopted as a safer alternative to typical agents. It combines potent D2 antagonism with significant 5-HT2A blockade — a profile that reduces EPS compared to first-generation agents. However, risperidone's EPS profile is dose-dependent in a clinically important way: at higher strengths, D2 receptor occupancy increases to the point where it begins to behave more like a typical antipsychotic in terms of movement side effects.

Risperidone is notable for causing the strongest prolactin elevation among atypical antipsychotics — a consequence of D2 blockade in the pituitary gland, which is not protected by the blood-brain barrier. Elevated prolactin can cause galactorrhea (inappropriate milk production), gynecomastia in males, menstrual irregularities, and sexual dysfunction. This is a consideration that should be discussed with the prescriber, particularly for younger patients or those concerned about reproductive effects.

Available in 0.25mg, 0.5mg, 1mg, 2mg, 3mg, and 4mg tablets; oral solution; orally disintegrating tablets; and Risperdal Consta — a biweekly long-acting injectable — as well as paliperidone (the active metabolite of risperidone), which is available as the once-monthly Invega Sustenna and once-quarterly Invega Trinza LAIs.

Quetiapine is one of the broadest-used antipsychotics, partly because of its FDA approvals for schizophrenia, bipolar disorder (manic episodes and bipolar depression), and as an adjunctive treatment for major depressive disorder — but significantly also because of its widespread off-label use. Its strong H1 antihistamine activity makes it highly sedating, particularly at lower amounts where D2 occupancy is modest and the sedating properties dominate the clinical picture.

This sedating quality has made quetiapine one of the most commonly prescribed medications off-label for insomnia and anxiety — a practice that is highly controversial. Critics point out that exposing patients without psychotic disorders to antipsychotic-class side effects (metabolic changes, the theoretical risk of tardive dyskinesia, next-day sedation impairing driving) for a non-psychotic indication represents a risk-benefit imbalance that may not be justified when other evidence-based options exist. Quetiapine does not carry FDA approval for insomnia or anxiety.

At higher antipsychotic-range strengths, metabolic effects — including moderate weight gain and lipid changes — become clinically relevant. Available in immediate-release 25mg, 50mg, 100mg, 150mg, 200mg, 300mg, and 400mg tablets; and as extended-release XR formulation (50mg, 150mg, 200mg, 300mg, 400mg). There is no LAI formulation.

Aripiprazole has a mechanism that sets it apart from every other antipsychotic: rather than blocking the D2 receptor, it is a partial agonist. It binds to the D2 receptor and produces a partial response — activating it just enough to prevent endogenous dopamine from producing a full response, but not so much as to fully stimulate the receptor. In brain regions where dopamine is excessively active (the mesolimbic pathway in psychosis), this partial activation effectively reduces signaling. In regions where dopamine is underactive, aripiprazole's partial agonism provides some stimulation.

This mechanism produces a distinct clinical profile. Aripiprazole causes minimal weight gain — it is often considered weight-neutral or associated with only trivial weight changes. Sedation is low, which patients often prefer, though some find it activating in a way that can worsen anxiety or insomnia early in treatment. EPS risk is low and QTc prolongation is minimal. It does not substantially elevate prolactin.

Aripiprazole is FDA-approved for schizophrenia, bipolar I disorder, MDD (as adjunct), irritability associated with autism spectrum disorder, and Tourette's disorder. Available in 2mg, 5mg, 10mg, 15mg, 20mg, and 30mg tablets; orally disintegrating tablets; oral solution; short-acting IM injection; and Abilify Maintena — a once-monthly long-acting injectable. A newer subdermal implant (Abilify MYCITE with a sensor, and Aristada subcutaneous) also exists.

Ziprasidone combines D2 and 5-HT2A antagonism with activity at several other serotonin and norepinephrine receptors. It is considered relatively weight-neutral — a meaningful advantage — but this benefit is limited in practice by its two most important clinical caveats: significant QTc prolongation and a strict food requirement.

Ziprasidone causes the most QTc interval prolongation of any atypical antipsychotic. A prolonged QTc interval increases the risk of potentially fatal cardiac arrhythmias, particularly when combined with other QTc-prolonging agents (of which there are many in common use). Patients with pre-existing cardiac conditions, electrolyte abnormalities, or who require multiple QTc-prolonging medications may not be good candidates. An ECG before and during treatment is often recommended.

The food requirement is not minor: ziprasidone's bioavailability approximately doubles when taken with food of sufficient caloric content. Taking it on an empty stomach significantly reduces the effective amount absorbed. Available in 20mg, 40mg, 60mg, and 80mg capsules, as well as a short-acting IM formulation for acute agitation.

Lurasidone is a newer atypical antipsychotic that has carved a useful clinical niche. Its receptor profile includes D2 and 5-HT2A antagonism (standard for atypicals), but it also has strong antagonism at the serotonin 5-HT7 receptor — a target implicated in mood regulation and cognitive function. This may contribute to its particular effectiveness in bipolar depression, for which it is FDA-approved as both monotherapy and as an adjunct to lithium or valproate.

Lurasidone's metabolic profile is favorable. Weight gain is minimal — among the lowest of the atypicals — and it has no clinically meaningful effect on QTc interval. These properties make it a preferred option when metabolic and cardiac safety are priorities. EPS risk is low to moderate, and sedation (primarily related to H1 activity) is moderate.

Like ziprasidone, lurasidone has a significant food requirement — but lurasidone's is more precisely defined: it must be taken with at least 350 calories of food. Without adequate food intake, bioavailability drops substantially. Available in 20mg, 40mg, 60mg, 80mg, and 120mg tablets. There is currently no LAI formulation.