Restarting Clozapine After Neutropenia or Myocarditis: Rechallenge Evidence

The Clozaril label says "Generally, do not rechallenge patients with CLOZARIL in those who experienced severe neutropenia". It allows that for some patients "the risk of schizophrenia exacerbation from not restarting CLOZARIL treatment may be greater than the risk of neutropenia reoccurrence", such as patients with no other treatment options.¹ In a 2018 review of 259 published cases, rechallenge was successful in 128 of 203 patients after neutropenia (63.0%) and in 3 of 17 after agranulocytosis (17.7%).²

Those counts come from uncontrolled case reports and series. Restarting clozapine after a blood or heart reaction is a specialist decision, made by the psychiatrist with a hematologist or cardiologist. Clozapine stopped for one of these reactions is never restarted by the patient or family alone.

What the label says about restarting

The label defines severe neutropenia as an absolute neutrophil count (ANC) below 500/μL. Its dosing table says to discontinue treatment and recommend a hematology consultation, and then: "Once ANC ≥ 1500/μL, if the benefits outweigh the risks of restarting treatment, resume treatment and obtain ANC tests according to the frequency for patients who initiate treatment".¹ The thresholds are on the clozapine blood tests page.

On myocarditis, two passages of the label differ. The boxed warning says "Generally, patients with CLOZARIL-related myocarditis or cardiomyopathy should not be rechallenged with CLOZARIL". Section 5.5 gives the same general rule and adds that "if the benefit of CLOZARIL treatment is judged to outweigh the potential risks of recurrence, the clinician may consider rechallenge with CLOZARIL in consultation with a cardiologist".¹ Neither passage mentions a rechallenge protocol, and sections 5.1 and 5.5 do not mention lithium or G-CSF.

Ordinary missed days are covered on the missed doses page.

Published rechallenge outcomes by adverse event

Source: Manu and colleagues.^{3, 2} The 2018 update added 121 cases to the 2012 set.

Both reviews concluded that rechallenge can be considered after neutropenia and neuroleptic malignant syndrome, and both advised against it after agranulocytosis or myocarditis.^{3, 2} In a 2023 review of rechallenge or continuation supported by colony-stimulating factor, there was a trend toward higher success in case reports and series than in consecutive case series (84% and 60%, p = 0.065).⁴

How often neutropenia returns, and how severe it is

In a UK and Ireland cohort of 53 rechallenged patients, 20 (38%) had a further blood dyscrasia. In 17 of those 20 (85%) the second episode was more severe, in 12 (60%) it lasted longer, and in 17 (85%) it occurred more quickly. Twenty-nine of the 53 (55%) were still receiving clozapine, and no clear risk factor for a repeat episode was identified.⁵

A 2026 study used the Central Non-Rechallenge Database for the UK and Ireland. Of 4,719 registered patients, 1,296 (27%) were rechallenged, and 435 of those met the revised database criteria. Within 6 months, 29 of the 435 (7%) were re-registered after a further neutropenic episode, and 87 (20%) had less than 6 months of follow-up. The authors wrote that rechallenge can be successful "in a select group of patients".⁶

In a cohort of 19 patients, 4 (21%) had further neutropenia and 2 developed agranulocytosis. The authors judged that the first neutropenia "was unlikely to be related to clozapine in a substantial proportion of cases".⁷ In another series of 19 cases, one third had a new hematologic reaction, and it was less severe in 83%.⁸

A 2007 review held that clozapine may be restarted if non-clozapine causes of neutropenia are identified and eliminated, and that where clozapine is clearly the cause of agranulocytosis, rechallenge should not be considered outside "very exceptional circumstances".⁹

Lithium and G-CSF: small, uncontrolled evidence

Lithium is sometimes added to induce leukocytosis.⁹ In a retrospective analysis of 25 patients rechallenged with lithium, 1 (4%) had a second episode, against a UK national rate of 21.2%. The authors saw some evidence that lithium masked that patient's neutropenia.¹⁰ The 2007 review stated that "lithium does not protect against clozapine-related agranulocytosis".⁹

G-CSF is granulocyte colony-stimulating factor. A 2017 systematic review identified 32 reports of its use in rechallenge, and 75% (24) of the published cases remained on clozapine. One of 3 rechallenges in patients with a history of severe agranulocytosis ended in recurrence at week 9, and the authors wrote that "it is not possible to recommend this course of action for someone with a true clozapine agranulocytosis".¹¹ A second 2017 review found 76% of cases continuing clozapine at a median of 12 months, with success of 60% in cases co-prescribed lithium and 81% in those without.¹²

Clozapine rechallenge after myocarditis

A 2021 systematic review of published cases found 34 rechallenges, 22 successful (64.7%), with one fatality (2.9%). No demographic or clinical marker was significantly associated with success.¹³ A 2024 review identified 45 rechallenge cases, 31 successful.¹⁴ In one series of 3 patients, every rechallenge was discontinued for suspected myocarditis.¹⁵

In a 2026 Delphi consensus of 93 clinicians and academics, 80% of experts (N = 70) believed rechallenge after a first episode could be considered safe. The core group does not recommend it after cardiogenic shock or ventricular arrhythmias, or where cardiac status has not returned to baseline. The minimum investigations before rechallenge were high-sensitivity troponin, CRP, full blood count, vital signs, ECG, and an echocardiogram with a left ventricular ejection fraction of at least 50%.¹⁶

The consensus also recommended ceasing sodium valproate if clinically justifiable, waiting at least 8 weeks after the myocarditis resolves where possible and often at least 6 months after a more severe episode, and checking troponin and CRP twice weekly.¹⁶ A 2024 review proposed a minimum wait of 3 months.¹⁷ Monitoring during a first titration is on the clozapine myocarditis page.

Limitations of the evidence

This evidence has limitations. First, every success rate here comes from published cases, registries or selected cohorts with no control group. Second, one review found a trend toward higher success in case reports than in consecutive series, and the 53-patient cohort identified no clear risk factor for a repeat blood dyscrasia.^{4, 5} Nevertheless, the label and the reviews give the psychiatrist and the hematologist or cardiologist the same questions to weigh: whether clozapine caused the event, how severe it was, whether the blood count or cardiac status has returned to baseline, and whether any other treatment option remains.

In a crisis: call or text 988, or go to the nearest emergency department. A first episode of psychosis belongs in a hospital-based or coordinated specialty care program.

I start and manage clozapine in an outpatient practice in Chelsea, Manhattan. Details are on the schizophrenia care in NYC page. Clozapine should not be stopped, skipped, or restarted without the prescriber. This page is general information and does not replace an evaluation.

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