Clozapine (brand name Clozaril) is the most effective antipsychotic medication we have for treatment-resistant schizophrenia, yet it remains significantly under-prescribed. In our practice we believe more people deserve a fair trial of clozapine, because for the right person it can succeed where every other medication has failed. This page explains why we use it, how it works, how it is dosed and monitored, and the side effects we watch for.
When clozapine is considered
Clozapine is the recommended next step when schizophrenia has not responded adequately to at least two prior antipsychotic trials, including second-generation antipsychotics taken at a proper dose for a proper length of time. This is called treatment-resistant schizophrenia. The American Psychiatric Association also recommends clozapine when the risk of suicide remains high despite other treatment, based on landmark trials showing clozapine reduces suicidal behavior (Meltzer et al., 2003; American Psychiatric Association, 2020). The original studies established that clozapine works for people who did not improve on other antipsychotics (Kane et al., 1988), and about 40 percent of people with treatment-resistant illness respond to it (Siskind et al., 2017).
Why clozapine is under-prescribed
For decades, clozapine in the United States could only be prescribed through a strict federal monitoring program called the REMS (Risk Evaluation and Mitigation Strategy), which required registration and blood-count reporting before each prescription could be filled. That paperwork burden discouraged many clinicians from offering clozapine at all, and it is a major reason the medication has been under-used by psychiatric providers. Notably, much of Europe never used a REMS-style system, relying on routine blood monitoring without the same access barriers. In 2025 the U.S. Food and Drug Administration removed the Clozapine REMS entirely, concluding it was no longer needed to manage the risk and that it had been getting in the way of care (U.S. Food and Drug Administration, 2025; European Clozapine Task Force, 2025). Removing that barrier is an opportunity to offer clozapine to more of the people who could benefit.
How clozapine works — the receptors it affects
Clozapine is unique because it touches many different receptors in the brain, which is thought to explain both its effectiveness and its side effects. The table below lists the main receptors clozapine acts on and what each action means.
| Receptor | Clozapine’s action | What it means |
|---|---|---|
| Dopamine D2 | Weak, loosely bound antagonist | Reduces psychosis with very little stiffness or tremor and little rise in prolactin |
| Dopamine D4 | Antagonist (higher affinity than D2) | Thought to add to the antipsychotic effect while keeping movement side effects low |
| Dopamine D1, D3, D5 | Lower-affinity antagonist | Help fine-tune dopamine signaling |
| Serotonin 5-HT2A | Strong antagonist | Improves psychosis and mood and lowers movement side effects |
| Serotonin 5-HT2C | Antagonist | Linked to increased appetite and weight gain |
| Serotonin 5-HT1A | Partial agonist | May ease anxiety and depression and protect against movement effects |
| Serotonin 5-HT3 | Antagonist | Acts on the gut and may aid nausea and thinking |
| Serotonin 5-HT6 and 5-HT7 | Antagonist | May support cognition, mood, and sleep and body-clock rhythms |
| Histamine H1 | Antagonist | Causes drowsiness and contributes to weight gain |
| Alpha-1 adrenergic | Antagonist | Causes dizziness or low blood pressure on standing |
| Alpha-2 adrenergic | Antagonist | May boost norepinephrine signaling (mood and attention) |
| Muscarinic M1, M2, M3, M5 | Antagonist (anticholinergic) | Dry mouth, blurred vision, constipation, and a faster heart rate; can affect memory |
| Muscarinic M4 | Partial agonist | Paradoxically drives excess saliva and nighttime drooling |
Available forms and dosing
Clozapine comes as standard tablets (12.5, 25, 50, 100, and 200 mg), as orally disintegrating tablets that dissolve on the tongue without water (12.5, 25, 100, 150, and 200 mg), and as a liquid oral suspension (50 mg/mL) for people who have trouble swallowing pills. The dissolve-on-the-tongue form is helpful when swallowing is difficult or when there is concern about whether a pill was actually taken.
Clozapine must be started low and increased slowly. We typically begin at 12.5 mg once or twice on the first day and increase gradually over the following weeks, watching closely for drowsiness, dizziness, and other effects. Most people reach an effective dose somewhere around 300–450 mg per day, though the right dose is individualized and some people need more. We tailor the dose to two things: how you are doing, and your clozapine blood level. We always aim for the least amount of medicine that controls your symptoms. Clozapine can lead to meaningful improvement and even remission for people who had lost hope of getting better; we never promise remission, but lasting recovery is always our goal.
Checking your clozapine blood level
Clozapine has a target blood level that helps guide dosing, generally at least about 350 ng/mL for a good response. We check a level once you reach a steady dose, again when we change the dose, and any time your situation changes in a way that affects the level — for example, starting or stopping smoking (which strongly changes how the body clears clozapine), adding an interacting medication, or if we are concerned about side effects or missed doses. The blood sample is drawn as a trough, about 12 hours after your last dose (StatPearls; Tiihonen et al., 2017).
Blood-count (ANC) monitoring for neutropenia
Clozapine can rarely lower the infection-fighting white blood cells, a serious effect called severe neutropenia or agranulocytosis. The risk is concentrated in the first months of treatment — most serious cases occur within about the first 18 weeks — and it becomes negligible after roughly two years (Alvir et al., 1993; Oloyede et al., 2022). With the federal REMS program eliminated in 2025, how often we check blood counts is now guided by current best-practice evidence and shared decision-making rather than a one-size-fits-all federal rule. Contemporary expert consensus recommends a baseline ANC before starting, then weekly monitoring through the highest-risk early period — the first roughly 18 weeks (about 3 to 4 months) — after which monitoring is commonly reduced (often to about every four weeks), and routine ANC monitoring can generally be stopped after two years of stable, unremarkable results, with a periodic complete blood count thereafter (Northwood et al., 2025; American Society of Clinical Psychopharmacology, 2025; Oloyede et al., 2025; Veterans Health Administration, 2025). A normal ANC is 1,500 or above, and we adjust or pause clozapine if the count drops too low (cessation thresholds are individualized, generally around an ANC below 1,000). We always ask you to contact us right away for a fever or signs of infection so we can check a count promptly.
Side effects
The side effect that needs the most attention day to day is constipation. Clozapine slows the entire gut (called gastrointestinal hypomotility), and if it is ignored this can progress to serious, even life-threatening bowel blockage — so we treat constipation proactively, usually with a bowel regimen from the start, and we ask you to tell us if you are not having regular bowel movements (Every-Palmer et al., 2016; Every-Palmer et al., 2017; U.S. Food and Drug Administration, 2020). Other important effects include severe neutropenia (monitored with blood counts, above), inflammation of the heart muscle called myocarditis (most often in the first one to two months, with chest pain, shortness of breath, a fast heart rate, or fever), and seizures at higher doses. More common, usually manageable effects include sedation, weight gain and metabolic changes in blood sugar and cholesterol, excess saliva and nighttime drooling, a fast heart rate, dizziness on standing, and bedwetting. We monitor for these and manage them with you.
When to seek help right away
Contact us, or seek urgent care, for a fever or sore throat or other signs of infection (so we can check your blood count), for not having a bowel movement for several days or severe abdominal pain and bloating, for chest pain, shortness of breath, or a racing heart in the first weeks of treatment, or for a seizure. In an emergency, call 911.
Culturally grounded care
We care for the whole person, in the spirit of lōkahi — balance and harmony among the body, mind and emotions, spirit, ʻohana (family), and our connection to the ʻāina (land) and community. To mālama (care for and protect) you and to support living pono (in balance), we explain this medication carefully, decide together, welcome your ʻohana, and honor the values that ground you. Offering kōkua (help given freely, with aloha) is at the heart of how we practice.
Common questions about clozapine
When is clozapine used?
Clozapine is recommended for treatment-resistant schizophrenia, generally after at least two adequate antipsychotic trials have not worked, and when suicide risk remains high despite other treatment. It is the most effective option in these situations.
Why is clozapine under-prescribed?
A strict federal monitoring program (the REMS) created paperwork barriers that discouraged its use; much of Europe never used such a system. The U.S. FDA removed the REMS in 2025, which should improve access.
How often do I need blood tests on clozapine?
Current best practice is a baseline blood count, then weekly absolute neutrophil count (ANC) monitoring for the first roughly 18 weeks (about 3 to 4 months) when the risk is highest, reduced afterward (commonly to about every four weeks), with routine ANC monitoring generally stopped after two years of stable results. This replaces the older every-6-month REMS schedule. We also check clozapine blood levels when you reach a steady dose, after dose changes, and when something like a change in smoking affects the level.
What is the most important everyday side effect?
Constipation. Clozapine slows the gut, and untreated constipation can become dangerous, so we start a bowel regimen early and ask you to report any change in bowel habits.
This page is general health education from OhanaPsych. It is not a substitute for a personal evaluation by a qualified clinician who knows your situation, and reading it does not create a provider–patient relationship. Do not start, stop, or change any medication without talking with your clinician. In an emergency, call 911.
George Mackel, MSN, APRN, NP-C, PMHNP-BC, CARN-AP — President & Owner
References
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American Psychiatric Association. (2020). The American Psychiatric Association practice guideline for the treatment of patients with schizophrenia. American Journal of Psychiatry. https://psychiatryonline.org/doi/10.1176/appi.ajp.2020.177901
European Clozapine Task Force. (2025). The time has come for revising the rules of clozapine blood monitoring in Europe: A joint expert statement. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC11822956/
Every-Palmer, S., Nowitz, M., Stanley, J., Grant, E., Huthwaite, M., Dunn, H., et al. (2016). Clozapine-treated patients have marked gastrointestinal hypomotility, the probable basis of life-threatening gastrointestinal complications: A cross-sectional study. EBioMedicine, 5, 125–134. https://www.sciencedirect.com/science/article/pii/S2352396416300536
Every-Palmer, S., Ellis, P. M., Nowitz, M., Stanley, J., Grant, E., Huthwaite, M., & Dunn, H. (2017). Clozapine-induced gastrointestinal hypomotility: A 22-year bi-national pharmacovigilance study of serious or fatal ‘slow gut’ reactions. CNS Drugs, 31(8), 699–709. https://pubmed.ncbi.nlm.nih.gov/28623627/
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Tiihonen, J., Mittendorfer-Rutz, E., Majak, M., Mehtälä, J., Hoti, F., Jedenius, E., et al. (2017). Real-world effectiveness of antipsychotic treatments in a nationwide cohort of 29,823 patients with schizophrenia. JAMA Psychiatry, 74(7), 686–693. https://www.semanticscholar.org/paper/9cfca764dc05d09f99c5c465ba83ea35ddd2f47f
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