Clozapine monitoring: what every new psychiatry doctor needs to know

Clozapine monitoring is the part of psychiatry most likely to land on a new doctor in the middle of the night. A nurse rings about an amber blood result, a missed dose, a temperature of 38.2°C, or a patient whose bowels haven't moved for five days. Each of these has a right answer, and some of them can't wait until morning.

Clozapine is the drug NICE recommends when schizophrenia hasn't responded to two different antipsychotics, at least one of them a second-generation drug other than clozapine. If you want the detail, I've linked what counts as treatment resistance. Clozapine often works when nothing else has. It also needs watching: the blood, the bowel, the heart and the dose.

Clozapine monitoring: reading the blood results

Clozapine can cause agranulocytosis, so everyone on it in the UK is registered with a monitoring service linked to their brand. The patient, the prescriber and the pharmacy are all registered. Each full blood count goes to the service, which grades it and sends back a colour.

The schedule sits in the UK section of the Clozaril SmPC (Summary of product characteristics):

• Weekly for the first 18 weeks.

• Every two weeks from week 18 to the end of the first year.

• Every four weeks after a year, once the counts have been stable.

• For four weeks after stopping, because the risk doesn't end with the last tablet.

Supply follows the bloods. An overdue count holds up the next supply. If the nurses tell you a blood test was missed, get it taken today.

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A result takes the colour of whichever count is worse. A white cell count of 4.1 with neutrophils of 1.8 is amber.

Amber

An amber result means the counts are low but not yet dangerous. Clozapine carries on, and the count is repeated twice a week until it settles or rises.

Red

A red result is the one you act on tonight. Stop clozapine straight away, and don't leave it for the morning team. Repeat the full blood count daily until it recovers, look for a source of infection, and tell your registrar. A fever with a red result is a medical emergency, so get the medical team in as you would for any possibly neutropenic patient.

Plenty of low counts aren't caused by clozapine at all. Benign ethnic neutropenia, common in people of African or Middle Eastern descent, gives a lower baseline count. Starting clozapine in someone with it needs a haematologist's agreement, and the Maudsley now recommends genetic testing for it in everyone starting clozapine.

Missed doses and the 48-hour rule

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What catches people out: the last dose on the chart isn't always the last dose taken. After leave, or a move from another ward or hospital, ask the patient directly when they last swallowed one.

Clozapine constipation: the side effect that kills

If you take one thing from this guide, make it this. Clozapine slows the whole gut. Constipation affects more than three in ten people on it. The Maudsley calls clozapine-induced gut hypomotility a far greater risk to life than agranulocytosis. When it's severe, around 20 to 30% of patients die, sometimes within hours of the first symptoms.

Patients rarely raise it, and self-report misses most cases. So you have to ask every time, in plain words:

• "When did you last open your bowels?"

• "Is that normal for you? Any straining, pain or bloating?"

• "Have you been sick, or had any leaking of loose stool?"

What to do

Fewer than three bowel movements a week, or any change from their usual pattern, means an abdominal examination. If there's no sign of obstruction, start a stimulant and a softener together. The Maudsley's example is senna 15 mg at night with docusate 100 mg three times a day, increased every 24 hours until the bowels move. Avoid bulk-forming laxatives such as ispaghula, which don't help this slow-transit constipation.

Then look at the rest of the chart. Hyoscine for drooling, procyclidine, opioids and tricyclics all add to the problem.

Myocarditis and fever in the first weeks

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Myocarditis

Myocarditis is an immune reaction to clozapine, and it comes early. The risk is greatest in the first two months, with a median onset around three weeks, and it can be fatal.

The symptoms are vague: fever, a flu-like illness, tiredness, breathlessness, chest pain, or a resting tachycardia that won't settle. Nausea and vomiting can be the only signs. The difficulty is that most people starting clozapine get a fast pulse anyway, and many get a fever.

So the Maudsley suggests baseline bloods, then CRP, troponin and a full blood count weekly for the first four weeks, with daily observations where possible. Your trust will have its own version.

Two results mean stopping clozapine and involving cardiology that day: a CRP above 100 mg/L, or a troponin more than twice the upper limit of normal.

If fever and tachycardia come with a raised CRP or troponin below those levels, the advice is daily CRP and troponin until the picture is clear.

A patient who has had CONFIRMED clozapine myocarditis should not be given it again.

Fever

Fever above 38°C is common in the first three weeks and is usually a harmless inflammatory response. On call, though, you can't assume it's the harmless kind. I work through four causes:

• Neutropenia. Send an urgent full blood count.

• Myocarditis. Check the pulse, an ECG, CRP and troponin, especially in the first two months.

• Infection, especially pneumonia. Pneumonia is a common cause of death on clozapine, partly from aspirated saliva.

• Neuroleptic malignant syndrome. High fever with rigidity, confusion or swinging blood pressure. I've covered it in the guide to extrapyramidal side effects and NMS.

Paracetamol is reasonable for the temperature, alongside the bloods, never instead of them. Tell your senior about any fever on clozapine, because slowing the titration is often part of the plan.

Infection changes the level too. Inflammation slows the enzyme that clears clozapine, and people often stop smoking when they're unwell. The MHRA advises a clozapine level when someone has pneumonia or another serious infection.

Hypersalivation and other everyday side effects

Drooling, worst at night, affects most people on clozapine to some degree. It's embarrassing and a common reason people stop the drug. It matters medically too, because aspirated saliva contributes to pneumonia.

Simple measures come first: a towel over the pillow, an extra pillow to raise the head, chewing gum in the day. A lower dose may help. No drug is licensed for this. Hyoscine is widely used, but it's anticholinergic and will worsen constipation, so I'd leave that choice to the team rather than start it overnight.

Sedation, dizziness on standing, weight gain and night-time wetting are common too.

Seizures are related to dose and plasma level - can be managed by AED especially when using high doses of clozapine. Sudden jerks, or knees that buckle, can be myoclonus, which may come before a seizure. Report them to your senior.

Clozapine and smoking: why a cigarette changes the dose

Tobacco smoke speeds up CYP1A2, the liver enzyme that clears most clozapine. It's the smoke (products of combustion) that does it, not the nicotine. Smoking can cut clozapine levels by up to 50%, and as few as two to five cigarettes a day may be enough. The textbook page on pharmacokinetic interactions has the mechanism.

Now think about admission. Someone who smoked heavily at home arrives on a smoke-free ward. Over the next few days the enzyme slows down and the level climbs on the same dose. Nicotine patches and vaping don't prevent it, because they contain no smoke.

A rising level looks like the side effects turned up: drowsiness, more drooling, constipation, myoclonus and, at worst, seizures. The Maudsley plan when someone stops smoking:

1. Take a clozapine level before they stop, or as soon as you find out.

2. Reduce the dose gradually over a week to about 75% of the original.

3. Repeat the level a week after they stop, and expect further changes.

The reverse happens on leave or discharge. If they start smoking again, the level falls and relapse becomes a risk. Take a level, return to the smoking dose over a week, and recheck.

So I ask about it at every review: "Are you smoking more, less or about the same as last week? Any vaping?"

In the CASC, clozapine explanation stations often hide a smoking question. If the patient mentions quitting, tell them to let the team know first so the dose can be adjusted safely. The CASC notes on psychotropic medicines cover the rest of that station.

Before you prescribe anything new

Clozapine interacts with a lot. Before I add anything on call, I run through four questions:

• Will it raise the level? Ciprofloxacin and fluvoxamine block CYP1A2 and can push the level up sharply. Hormonal contraceptives and caffeine raise it too.

• Will it lower the level? Phenytoin, rifampicin and omeprazole can all reduce it.

• Will it hit the bone marrow? The SmPC says not to combine clozapine with drugs known to suppress the marrow. It names carbamazepine, chloramphenicol, co-trimoxazole, cytotoxics and depot antipsychotics.

• Will it add to sedation, constipation or low blood pressure? Benzodiazepines carry a rare risk of collapse and respiratory arrest with clozapine, most often when the two are started together. Anticholinergics add to constipation, and antihypertensives to hypotension.

What I'd do before your first on-call

On your first day, find out three things. Which patients on your wards are on clozapine, and when did each one start? Where do blood results come through out of hours, and who acts on a red? How do you reach the on-call pharmacist?

Then open every clozapine call with the same questions. When did they start clozapine? When did they last swallow a dose? When did they last open their bowels? Are they smoking? Those four answers will point you to the right part of this guide most nights.

Raj

References

1. National Institute for Health and Care Excellence. Psychosis and schizophrenia in adults: prevention and management (CG178). 2014, updated 2026. https://www.nice.org.uk/guidance/cg178

2. Viatris. Clozaril 100 mg tablets: summary of product characteristics. 2026. https://www.medicines.org.uk/emc/product/10290/smpc

3. Medicines and Healthcare products Regulatory Agency. Clozapine: reminder of potentially fatal risk of intestinal obstruction, faecal impaction, and paralytic ileus (Drug Safety Update). 2017. https://www.gov.uk/drug-safety-update/clozapine-reminder-of-potentially-fatal-risk-of-intestinal-obstruction-faecal-impaction-and-paralytic-ileus