Clozapine. What does it do

Clozapine is a powerful drug for a narrow group of people, with a serious list of harms that require constant monitoring. It is not a first-choice treatment.

Clozapine is an antipsychotic reserved for people with schizophrenia who have not gotten better after trying at least two other drugs. The landmark 1988 trial that put it on the map was funded by the manufacturer, Sandoz, and compared clozapine to chlorpromazine in 268 treatment-resistant patients [7]. After six weeks, 30% of the clozapine group improved versus 4% of the chlorpromazine group [7]. That is a real effect, and it has been replicated. A large European observational study (SOHO) found that over three years, clozapine had the lowest rate of discontinuation of any antipsychotic studied, with a hazard ratio of 0.82 compared to olanzapine [3]. It also reduces tardive dyskinesia, the involuntary movement disorder caused by older antipsychotics, by about 52% at six months and 87% at five years [1, 2].

The mechanism is messy. Clozapine blocks a wide range of receptors: dopamine D2 (weakly), serotonin 5-HT2A, histamine H1, muscarinic M1, and alpha-adrenergic receptors. That broad profile is why it works when other drugs fail, and also why its side effect list is long.

The harms are not minor. The most feared is agranulocytosis, a collapse of white blood cells that leaves the body defenseless against infection. The incidence is about 0.7 to 0.8% [4]. That is why anyone on clozapine must have weekly blood tests. The risk of myocarditis, inflammation of the heart muscle, is about 0.7 to 1.2% in the first two months of treatment, mostly in Australia where the data is best [5, 6]. Cardiomyopathy, a weakening of the heart, occurs in about 0.02 to 0.1% of patients [5]. Other common side effects include severe drooling (71% of patients in one study), constipation (71%), weight gain, sedation, and seizures at higher doses [8].

The evidence for clozapine's benefit in treatment-resistant schizophrenia is solid, but it comes almost entirely from industry-funded trials and observational studies that compare it to other drugs, not to no treatment. The long-term safety data on heart and metabolic damage is thin. The drug works, but the burden of proof for starting it is high, and the monitoring burden for staying on it is higher.

My call: clozapine is a genuinely effective drug for a small, severely ill population, but its harms are serious and the evidence base is funded by the manufacturer. It should be used only when other options have failed, and only with rigorous monitoring. Confidence: moderate.