Clozapine Drooling: Why It Happens and What the Treatment Trials Show

Creator: Ryan S. Sultan, MD
Published: 2026-11-12

By Ryan S. Sultan, MD
Psychiatrist · Columbia University Irving Medical Center
Published November 12, 2026 · 5-minute read

Drooling, also called sialorrhea or hypersalivation, is common on clozapine, and published estimates vary widely. A Cochrane review put it at 30% to 80%, a trial report at 31% to 57%, and in one clinic survey 92% of participants experienced it.^{1, 2, 3}

The reviews don't establish any one treatment. The Cochrane review concluded that there were insufficient data to confidently inform clinical practice, and a 2023 network meta-analysis of randomized trials described its own findings as low-confidence.^{1, 4}

How common it is, and when it's worst

In the clinic survey, drooling at night was more prevalent than drooling during the day (85% versus 48%). Daytime drooling was severe in 18% of cases, and hypersalivation had at least a moderate impact on quality of life for 15% of participants.³

A study that measured salivary flow weekly in 17 patients found an increase from baseline after clozapine was started, with a significant increase between the second and third weeks followed by a plateau. Flow rate didn't correlate with clozapine dose.⁵ A 2022 systematic review of adverse reactions and plasma levels grouped sialorrhea with the reactions that were "rather unrelated" to the level.⁶

Why clozapine causes drooling

The mechanism is poorly understood.⁷ Drooling is thought to be related to norclozapine, clozapine's active metabolite, stimulating M1 muscarinic receptors in the salivary glands. Clozapine and norclozapine have both muscarinic agonist and antagonist properties, and the net clinical effect depends on the balance between them.⁸

The same paper describes clozapine as predominantly anticholinergic, which is thought to be primarily responsible for constipation.⁸ Both accounts are hypotheses.

What the treatment trials show

Drooling on clozapine is usually managed with anticholinergic medications used off-label.⁹ The four placebo-controlled trials below are small.

Placebo-controlled trials of four treatments for drooling
Treatment	What the trial found
Glycopyrrolate	Crossover trial in 32 patients with drooling at night, with 6-day exposure periods. At the randomized 1 mg dose, 18.8% improved compared with 6.3% on placebo, which was not a significant difference (P = 0.289). In an optional open-label extension at 2 mg, 43.5% improved (P = 0.039 against placebo).
Ipratropium	Crossover trial in 20 patients, with 2-week phases. There were 9 responders (45%) on ipratropium and 8 (40%) on placebo, and no significant reduction on any of three rating scales.
Sublingual atropine	Trial in 21 patients given a single 600-μg dose or placebo, with saliva measured 2 hours later. Saliva secretion fell significantly more with atropine (mean difference -57.21%, 95% CI -104.30 to -10.11). Standing pulse rate also decreased in the atropine group.
Botulinum toxin B	Trial in 9 patients with severe drooling, 4 with antipsychotic-induced drooling and 5 with Parkinson's disease, given injections of toxin or placebo into the salivary glands. The reduction lasted 8 to 16 weeks after a single injection, and no patient reported side effects.

Sources: the four trials.^{9, 10, 11, 12} Doses are the ones tested. The choice of treatment and dose is the prescriber's.

The network meta-analysis pooled 33 randomized trials with 1,958 participants, all in clozapine-induced drooling. Twelve drugs outperformed placebo on response, among them metoclopramide, propantheline, diphenhydramine, amisulpride, and atropine, while glycopyrrolate and ipratropium did not.⁴ For sublingual atropine and botulinum toxin, the table shows the one placebo-controlled trial of each found in a November 2026 literature search.

For drooling at night, neither benzamides nor atropine outperformed placebo in that analysis.⁴ An earlier meta-analysis of 19 studies found improvement with propantheline, diphenhydramine, chlorpheniramine, and benzamide derivatives, and its authors noted that the studies were limited by poor quality.¹³ Of the 15 trials in the Cochrane review, 14 were conducted in China, and the quality of reporting was poor.¹

The trade-off with constipation

Clozapine causes constipation, and the FDA tells prescribers to avoid co-prescribing it with other anticholinergic medications that can cause gastrointestinal hypomotility.¹⁴ I've written separately about clozapine constipation.

Several sources point to added risk. In the 19-study meta-analysis, propantheline increased rates of constipation, with a number needed to harm of 9 (95% CI 4.2 to 204.1).¹³ In a registry study of patients with schizophrenia, anticholinergic medications as a class were associated with ileus (odds ratio 1.48) and with fatal ileus (odds ratio 5.88, 95% CI 1.47 to 23.58).¹⁵ A 2025 paper states that systemic anticholinergics carry elevated risks of constipation, bowel impaction, or ileus, and that topical anticholinergics show limited efficacy.⁸

In the network meta-analysis, however, active treatments didn't differ significantly from placebo on constipation, with low confidence in the findings.⁴ The Cochrane review found that adverse effects were poorly recorded.¹ How much any one drooling treatment adds to bowel risk remains uncertain.

What to raise with the prescriber

Whether the drooling is mainly at night or also during the day, and how much it interferes
Current bowel habits, since several of the options are anticholinergic
Other medications already being taken that are anticholinergic or constipating
What evidence supports the option being suggested, and how its effect will be judged

These treatments are started, stopped, and adjusted by the prescriber, and so is the clozapine dose. The other side effects are covered on the clozapine side effects page.

Limitations of the evidence

This evidence has limitations. First, the four placebo-controlled trials in the table enrolled 9 to 32 people, and the atropine trial tested a single dose. Second, adverse effects were poorly recorded, so the bowel risk of each treatment is uncertain. Nevertheless, the reviews agree that no option is well established, and the FDA advice on co-prescribing anticholinergic medications applies to several of the options.

Whether to treat the drooling, and with what, is decided with the prescriber, with bowel function part of that decision.

Frequently asked questions

Why does clozapine cause drooling?

The mechanism is poorly understood. Drooling is thought to be related to norclozapine, clozapine's active metabolite, stimulating M1 muscarinic receptors in the salivary glands.^{7, 8}

How common is drooling on clozapine, and is it worse at night?

A Cochrane review put it at 30% to 80%, and in one clinic survey 92% of participants experienced it. In that survey, drooling at night was more prevalent than drooling during the day, 85% versus 48%.^{1, 3}

Does glycopyrrolate work for clozapine drooling?

In a crossover trial of 32 patients with drooling at night, 18.8% improved at the randomized dose compared with 6.3% on placebo, which was not a significant difference. A higher dose did better than placebo in an open-label extension, and in a 2023 network meta-analysis glycopyrrolate did not outperform placebo. The choice of treatment and dose is the prescriber's.^{9, 4}

Do atropine drops under the tongue help clozapine drooling?

The placebo-controlled trial of sublingual atropine found in a November 2026 literature search was small: 21 patients given a single dose, with saliva measured 2 hours later. Saliva secretion fell significantly more with atropine than with placebo. Whether to use it is the prescriber's decision.¹¹

Do drooling medications make clozapine constipation worse?

Some may. The FDA tells prescribers to avoid co-prescribing clozapine with other anticholinergic medications that can cause gastrointestinal hypomotility, and propantheline increased rates of constipation with a number needed to harm of 9. In the network meta-analysis, active treatments did not differ significantly from placebo on constipation, with low confidence in the findings.^{14, 13, 4}

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.