Clozapine works when other medicines fail. Why is it so rarely used?
Guidelines recommend it after two other antipsychotics have not helped. Many people wait years for it, or never get it.
Published Updated 4 min read
Key points
- Between a quarter and a third of people with schizophrenia do not improve enough on standard antipsychotics. This is called treatment-resistant schizophrenia.1
- Clozapine is the only medicine approved for it. In studies, around four in ten people with treatment-resistant schizophrenia improve on it.1
- It needs regular blood tests. In about 1 in 100 people, it causes a severe drop in white blood cells.2
What makes clozapine different
Most antipsychotics (opens in a new tab) block dopamine D2 receptors (opens in a new tab) strongly. Clozapine blocks them only weakly: it binds to D2 about 70 times more weakly than haloperidol, an older antipsychotic.3 In brain scans at the doses used in practice, clozapine occupies 16 to 68 in 100 D2 receptors. Other antipsychotics start to work once they occupy more than about 65 in 100.4,5
Instead, clozapine binds more strongly to receptors for noradrenaline, histamine, serotonin and acetylcholine, and to another dopamine receptor, D4.3
Where these receptors are
D2 receptors are concentrated in the striatum, deep in the brain, which helps control movement. D4 receptors are found mainly in the prefrontal cortex, the hippocampus and the thalamus, and hardly at all in the striatum.6,7
When the D4 receptor was discovered in 1991, clozapine’s strong hold on it looked like the explanation for its success.8 But a medicine that blocks only D4 did not help people with schizophrenia.9 Clozapine’s main breakdown product, norclozapine, also activates M1 receptors for acetylcholine, which no other antipsychotic does.10 Which of these actions explains why clozapine works when other medicines have not is still unknown.
It is the only medicine approved for treatment-resistant schizophrenia, and it is often offered last.
Who it is for
UK guidelines recommend offering clozapine when symptoms have not improved enough after two different antipsychotics, each taken at an adequate dose for long enough.11 Experts define “enough” and “long enough” in a consensus published in 2017.12 Around 40 in 100 people with treatment-resistant schizophrenia improve on clozapine.1
Clozapine and movement
Blocking D2 receptors in the striatum is why older antipsychotics cause stiffness, tremor and restlessness. In a brain-scan study of haloperidol, these became likely once more than about 78 in 100 D2 receptors were occupied.5 Clozapine occupies far fewer, and causes these problems much less often than older antipsychotics.13 They do still occur: tremor, stiffness and restlessness affect between 1 and 10 in 100 people taking it.13
Parkinson’s disease shows the link from the other side. It is treated with medicines that raise dopamine, and these can bring on psychosis; the disease itself also makes psychosis more common over the years.14 Most antipsychotics would make movement worse. In a trial, low-dose clozapine eased psychosis in people with Parkinson’s without worsening their movement, and it reduced their tremor.15
After years on antipsychotics, some people develop tardive dyskinesia: involuntary movements, often of the mouth and face, that can last. With clozapine it has been reported only very rarely, and people who had developed it on other antipsychotics improved after switching to clozapine.13,16
Why blood tests are needed
About 4 in 100 people on clozapine develop a drop in neutrophils, the white blood cells that fight infection, and about 1 in 100 a severe drop. The risk is highest in the first months and becomes very small after a year. Deaths are very rare: about 1 in 7,700 people prescribed clozapine.2 Regular blood tests catch the drop early, so the medicine can be stopped in time.
Since 2025, the tests are less frequent in Europe: weekly for the first 18 weeks, monthly to the end of the first year, every 12 weeks in the second year, and once a year after that.17,13
Weighing the side effects
Clozapine’s list of possible side effects is long, and families often read it before anything else. Worry about side effects is one of the most common reasons it is not started. Staff in one London mental health trust named patients’ concerns about side effects as the most frequent barrier.18 Among hospital patients who could have been offered clozapine, 43 in 100 said those concerns alone would make them refuse it.19
That hesitation is understandable: the list is longer than for haloperidol, aripiprazole or olanzapine. Most of the extra entries are rare side effects. Those that affect more than 1 in 100 people are about as many as with haloperidol or olanzapine. Counted from each medicine’s product information and rounded to the nearest 10:
| Medicine | Possible side effects listed | Of these, affecting more than 1 in 100 people |
|---|---|---|
| Clozapine13 | 120 | 30 |
| Haloperidol20 | 100 | 30 |
| Aripiprazole21 | 100 | 20 |
| Olanzapine22 | 70 | 30 |
Views tend to change once people take it. In studies of people on clozapine and their families, between 30 and 80 in 100 patients and more than 90 in 100 carers saw it positively. Most felt the benefits outweighed the side effects and the blood tests.23
The side effects that most often make people stop are not the rare dangerous ones, but everyday ones: producing too much saliva and sleepiness. Both patients and families said they had not been told enough about them.23 The serious risks, such as a severe drop in white blood cells or inflammation of the heart, are uncommon, and blood tests and check-ups are there to catch them early.2,13
Why it is underused
The blood tests, the paperwork that came with them, and worry about side effects make clozapine harder to start than other medicines.18 In the United States, the regulator removed its special monitoring programme in 2025, but blood tests are still advised.24
What we don’t know yet
- Why clozapine works when other antipsychotics have not.
- How to predict who will respond before starting it.
