Most psychiatric medications are dosed based on what works in trials and what the patient tolerates. Blood levels usually don't enter the conversation. Antipsychotics are partly different. For clozapine the level matters a lot. For several others — olanzapine, perphenazine, haloperidol — the level can clarify a confusing clinical picture. For most others the test exists but is rarely used.
This piece is about when therapeutic drug monitoring (TDM) helps, what the targets are, and what changes blood levels in real life.
Plasma-level monitoring is standard for clozapine (target generally ≥350 ng/mL for response) and useful for several other antipsychotics in specific situations — adherence questions, unexpected response, suspected drug interactions, or extreme side effects.
Why blood levels matter even when the dose is the same
Two people on the same milligram dose can have plasma concentrations that differ tenfold. Reasons include:
- Genetic differences in cytochrome P450 enzymes (CYP1A2, 2D6, 3A4) that metabolise the drug
- Smoking — induces CYP1A2 and lowers clozapine and olanzapine levels substantially
- Coffee and caffeine — inhibit CYP1A2 and raise clozapine levels
- Drug interactions — many antibiotics, antifungals, antidepressants, and OTC products affect metabolism
- Liver and kidney function
- Adherence — missed doses produce levels that don't match the prescription
- Inflammation and infection — can transiently increase clozapine levels
Clozapine: where TDM is essential
The Schulte threshold of approximately 350 ng/mL for clozapine is one of the most established findings in psychopharmacology. Below this level, response rates are much lower; above it, response rates rise. Above roughly 600 ng/mL, side effects (especially seizures) increase without much added efficacy.
Practical points:
- Levels are drawn as a trough — typically 12 hours after the last dose, before the next dose.
- Steady-state takes about 5–7 days at a stable dose.
- Norclozapine (the main metabolite) is often reported alongside clozapine. Ratios outside 0.4–0.7 can suggest unusual metabolism, smoking changes, or adherence issues.
- A patient with a clozapine level of 200 ng/mL on 500 mg/day is essentially under-treated, regardless of dose. Either the dose needs to go up or something is interfering.
What changes clozapine levels day to day
Smoking
Tobacco smoke (the combustion products, not nicotine) induces CYP1A2. People who smoke usually need higher clozapine doses for the same level. The clinical consequence: if a patient quits smoking suddenly, clozapine levels can rise substantially within days. Hospital admissions are a common scenario — the patient enters a non-smoking ward, levels climb, and side effects worsen. The dose often needs to be reduced. See smoking and schizophrenia.
Caffeine
Coffee inhibits the same enzyme. Big swings in caffeine intake can move levels noticeably. Most clinical guidance suggests avoiding both heavy caffeine intake and abrupt changes in coffee consumption.
Infection and inflammation
Acute illness, particularly with fever, can transiently raise clozapine levels — sometimes substantially. A patient with a UTI, pneumonia, or COVID-19 can develop temporary side effect amplification. This is a recognised reason to recheck levels and sometimes lower the dose during acute illness.
Drug interactions
Fluvoxamine and ciprofloxacin are notable CYP1A2 inhibitors that can dramatically raise clozapine levels. Carbamazepine and rifampin lower levels. CBD can affect levels through CYP enzyme inhibition.
Other antipsychotics where TDM helps
Olanzapine
Reasonable evidence supports a target plasma level around 20–80 ng/mL for response. Smoking has a similar effect on olanzapine as on clozapine. Useful in suspected non-response or side-effect outliers.
Haloperidol
Therapeutic range often cited as 5–15 ng/mL. Useful when a patient has significant EPS at low oral doses or non-response at higher doses.
Perphenazine and fluphenazine
TDM is available and used in some settings, particularly for LAI versions where dosing is intermittent.
LAI versions of risperidone, paliperidone, aripiprazole
Less commonly checked but available. More often used to confirm steady-state has been reached or to investigate suspected adherence or absorption issues.
What TDM does not do
- It doesn't tell you who will respond — it tells you whether the drug is at a level where response is plausible.
- It doesn't replace clinical judgement. A patient with a "perfect" level who isn't responding still isn't responding.
- It doesn't substitute for asking about adherence directly.
When to ask for a plasma level
- You're on clozapine and have not had a level checked in over 6 months
- You've started or stopped smoking
- You've started, stopped, or significantly changed dose of another medication
- You've had an acute illness with fever
- You're not responding as expected
- You're having unexpected side effects
- Adherence is in question
For clozapine, draw the level before the morning dose, ideally about 12 hours after the previous dose. A level drawn 2 hours after a dose will be misleadingly high.
The bigger picture
TDM is one of the cheapest, most informative tests in psychiatry, and one of the most underused. For clozapine especially, no rational dose adjustment can be made without it. For other antipsychotics, it earns its place when something unexpected is happening clinically.
This article is for educational purposes only and is not medical advice, diagnosis, or treatment. Always consult a qualified mental health professional. If you or someone you know is in crisis, call or text 988 in the US, or your local emergency number.