Kanna Clinical Studies: Every Published Human Trial
Seven published human studies, all of one branded extract, none in patients. Here is each one, side by side, with who paid for it.
People searching for kanna clinical trials usually want one thing: a straight count of the human evidence. The count is small. Seven published studies have tested kanna (Sceletium tortuosum) in people, all using the same branded extract, Zembrin, and all in healthy volunteers. This page lists every one in a single table, with design, size, dose, duration, outcomes and funding, and flags what we could not verify.
Seven published human studies of kanna exist, all testing the Zembrin extract (usually 25 mg) in healthy volunteers. They range from 16 to 60 participants and from a single dose to three months. Outcomes were brain scans, cognitive tests and rating scales, with mixed results. No published trial has studied kanna in patients, and several authors are linked to the extract's developer.
What are all the published human studies of kanna?
There are seven, listed here by publication date. Every study used Zembrin, a 2:1 standardized extract of Sceletium tortuosum [3]. Doses are mg of extract, not mg of plant.
| Study | Design | People | Extract and dose | Duration | What was measured | Funding and conflicts |
|---|---|---|---|---|---|---|
| Nell et al. 2013 [1] | Randomized, double-blind, placebo-controlled, parallel, one site (run in 2009) | 37 healthy adults (12 on 8 mg, 12 on 25 mg, 13 on placebo) | Zembrin 8 mg or 25 mg daily | 3 months | Safety only: vital signs, ECG, bloodwork, adverse events. No efficacy outcomes. | Funding statement not read (full text paywalled). Author N. Gericke is linked to HG&H, Zembrin's developer [3]. |
| Terburg et al. 2013 [2] | Double-blind, placebo-controlled crossover, fMRI | 16 healthy undergraduates aged 18 to 21 (8 female) | Zembrin 25 mg, single dose | One scan per condition, 5 to 9 days apart | Brain activity while viewing fearful faces; amygdala connectivity | Funded by H.L. Hall and Sons Ltd (South Africa) and the Netherlands Organization for Scientific Research. One author is an HG&H director [2]. |
| Chiu et al. 2014 [3] | Randomized, double-blind, placebo-controlled crossover, one site (Puerto Rico) | 21 enrolled, ages 45 to 61 (mean 54.6); five dropped out and analysis used completers | Zembrin 25 mg daily | 3 weeks on each arm, 3-week washout | Cognitive tests (CNS Vital Signs), HAM-D, adverse events | Funded by HG&H and PL Thomas (North American distributor). One author is an HG&H director and shareholder [3]. |
| Dimpfel et al. 2017 [4] | Randomized, double-blind, placebo-controlled, three-arm parallel | 60 healthy adults aged 50 to 80 (32 men, 28 women); numbers per arm not reported in the text we read | Zembrin 25 mg or 50 mg daily, or placebo | 6 weeks | EEG, six cognitive tests, mood (POMS), anxiety rating (HAM-A), sleep questionnaire | No funding statement. Author affiliations include HG&H and NeuroCode AG; paper declares no conflicts [4]. |
| Reay et al. 2020 [5] | Two double-blind, placebo-controlled, between-subject experiments | 20 young healthy volunteers across both | Zembrin 25 mg, single dose | One session each | Mood and anxiety ratings, heart rate, skin conductance during lab stress tasks | Funding not verified (full text not read). One author is listed with American Medical Holdings Inc. [5] |
| Hoffman et al. 2020 [6] | Randomized, placebo-controlled, two groups (blinding not stated in the abstract) | 60 recreationally trained adults aged 20 to 35 (48 men, 12 women) | Zembrin 25 mg daily | 8 days; tested 2 hours after the day-8 dose | Reactive agility, visual tracking, mood (POMS), alertness and energy ratings | Funding not verified. |
| Berry et al. 2021 [7] | Pilot, two groups, placebo-controlled | 16 untrained women | Zembrin; dose not stated in the abstract | 3 days around an arm-exercise session, tested at 48 hours | Perceived soreness, range of motion, mood (POMS), a blood marker of muscle damage | Funding not verified. |
"Not verified" means the paper's funding section was behind a paywall or blocked when we tried to read it, not that funding is absent. Doses are Zembrin only; other extracts are not interchangeable.
What should you know about each study?
Each study answered a narrow question, and the headline result of each is smaller than the title suggests. Brief notes follow; for plain-English takeaways see what the research shows.
- Nell 2013 was a safety trial. Vital signs, ECG, body weight and bloodwork did not differ from placebo. Headache was the most common adverse event and occurred more often on placebo. Some participants wrote unprompted diary remarks about stress coping and sleep, but no efficacy measure was collected [1].
- Terburg 2013 found lower amygdala reactivity to fearful faces after 25 mg, but only under low perceptual load, not high load. Task performance did not differ and the authors saw no general effect on subjective mood. One of 16 participants was excluded from the tasks for near-chance performance [2].
- Chiu 2014 reported better cognitive flexibility and executive function than placebo, with reported P values of 0.022 and 0.032 (the abstract and the results table list them in opposite order). Seven other measures, including memory, reaction time and processing speed, showed no significant difference. Nine domains were tested and the article does not describe correcting for that [3].
- Dimpfel 2017 reported many exploratory EEG and test comparisons. The anxiety-rating score fell significantly only at 50 mg (p = 0.03), most mood changes were not significant, and the sleep questionnaire showed no differences [4]. The paper is in an open-access journal and is not indexed in PubMed.
- Reay 2020 found no effect in the multitasking experiment and, in the public-speaking experiment, lower anxiety ratings before the speech plus a heart-rate interaction. The authors call it "first tentative behavioral evidence" [5].
- Hoffman 2020 found better performance on a complex reactive task under cognitive load, but no change in simple reaction tests, mood score or alertness ratings [6].
- Berry 2021 compared recovery markers after unaccustomed arm exercise. Lower perceived soreness and better range of motion were reported at 48 hours, and mood fell in both groups but less so with Zembrin. Exercise performance and the muscle-damage blood marker did not change. The authors call it a pilot [7]. It is a result about exercise soreness in untrained women, not a statement about pain.
Are there kanna trials still underway?
Yes, two registered trials have no published results that we could find as of October 29, 2026. Registration is a plan, not a finding.
| Registry ID | Sponsor | Design | Planned size and dose | Status on the registry |
|---|---|---|---|---|
| NCT05471804 [8] | Northumbria University, with HG&H Pharmaceuticals as collaborator | Randomized, placebo-controlled, parallel; ages 30 to 50; primary outcome subjective stress; secondary outcomes include sleep quality and cognitive function | 120 planned; Zembrin 25 mg daily for 8 weeks | Status unknown; last known "recruiting" in July 2022 |
| NCT07511088 [9] | PLT Health Solutions (industry; markets Zembrin) | Decentralized, randomized, placebo-controlled crossover; ages 18 to 60; anxiety ratings after self-identified stressful episodes | 250 estimated; 25 mg of a standardized Sceletium extract taken as needed | Recruiting (record updated April 2026) |
The Chiu trial's registry entry (NCT01805518) also lists 20 participants and a 45 to 65 age range, while the paper reports 21 enrolled [10]. Small mismatches like this are common, but they are why we report the paper's numbers and flag the difference.
Who paid for the research?
Much of it was supported by, or linked to, the companies that make and sell Zembrin. That does not make the findings wrong, but readers should weigh it.
Chiu 2014 states that it was financially supported by HG&H and PL Thomas, and its disclosure section says one author is an HG&H director and a trustee of a trust that holds a minor shareholding, even though the separate conflict-of-interest line says there are none [3]. Terburg 2013 was funded in part by a South African company, with one HG&H-director author [2]. Dimpfel 2017 includes HG&H-affiliated authors [4]. The two registered trials have an industry sponsor or an industry collaborator [8][9].
The Dutch co-funded imaging study and the two exercise studies have no HG&H-affiliated authors that we found; we could not confirm the exercise studies' funding. Replication by groups with no tie to the extract's owner is the clearest gap. For our own position: this site is published by Psilly Goose, which makes a kanna drink, and we say so on every page.
What is missing from the evidence?
Almost everything a consumer would want to know. The gaps are structural.
- No patients. All participants were healthy. A 2026 review of animal and clinical studies found a favourable safety profile and potential efficacy but called the evidence mixed, citing small samples and studies in clinically irrelevant populations [11].
- One extract. Every trial used Zembrin. Nothing here tells you how other extracts, teas or tinctures behave.
- No drinks, no combinations. No trial tested kanna in a beverage, or with kava, caffeine, L-theanine or lion's mane.
- Short and small. The longest study ran three months and the largest had 60 people.
- No medication interaction studies. We found no human study of kanna taken with serotonergic medicines. See kanna and SSRIs and is kanna safe.
How should you read these studies?
As early, hypothesis-generating work. Small trials with many outcomes will produce some significant results by chance, and a handful of findings (two of nine cognitive domains in Chiu, for example) are the kind that need replication before anyone leans on them.
The outcomes also mean different things. A reduced amygdala signal is a measurement, not a feeling. A lower rating on a lab stress task is not the same as day-to-day stress. For the mood and sleep results see what the research says about mood and sleep; for the lab-stress and cognition results see what it says about stress and focus. For the mechanism, see how kanna works.
None of this tells you what a dose in a drink will do. Doses in drinks range widely, so check the label against the studied 8 to 50 mg and see the dosage guide. This page does not recommend a dose.
How did we find these studies?
We searched PubMed (Sceletium, Zembrin, Mesembryanthemum tortuosum, mesembrine, kanna), Europe PMC and ClinicalTrials.gov on October 29, 2026, then read abstracts and, where open, full texts. The searches returned 144 PubMed records; the vast majority were chemistry, animal or lab papers, reviews, or unrelated. Dimpfel 2017 and Berry 2021 are not indexed in PubMed and were found through other routes (a company bibliography and a Crossref record). No case reports of kanna turned up. We may still have missed something, especially non-English or unindexed work. If you know of a study we omitted, tell us through the About page.
This page is general information, not medical advice. These statements have not been evaluated by the FDA. Kanna products are not intended for any disease or health condition. If you take serotonergic medicines or are pregnant or nursing, talk to a doctor before using kanna. kannadrinks.com is published by Psilly Goose, a kanna drink maker.
References
- Nell H, Siebert M, Chellan P, Gericke N. A randomized, double-blind, parallel-group, placebo-controlled trial of extract Sceletium tortuosum (Zembrin) in healthy adults. J Altern Complement Med 2013;19(11):898-904. PubMed
- Terburg D, et al. Acute effects of Sceletium tortuosum (Zembrin), a dual 5-HT reuptake and PDE4 inhibitor, in the human amygdala and its connection to the hypothalamus. Neuropsychopharmacology 2013;38(13):2708-16. PubMed Central
- Chiu S, et al. Proof-of-concept randomized controlled study of cognition effects of the proprietary extract Sceletium tortuosum (Zembrin) targeting phosphodiesterase-4 in cognitively healthy subjects. Evid Based Complement Alternat Med 2014;2014:682014. PubMed Central
- Dimpfel W, Gericke N, Suliman S, Chiegoua Dipah GN. Effect of Zembrin on brain electrical activity in 60 older subjects after 6 weeks of daily intake. World J Neurosci 2017;7(1):140-171. SCIRP
- Reay J, Wetherell MA, Morton E, Lillis J, Badmaev V. Sceletium tortuosum (Zembrin) ameliorates experimentally induced anxiety in healthy volunteers. Hum Psychopharmacol 2020;35(6):1-7. PubMed
- Hoffman JR, Marcus I, Dubnov-Raz G, Gepner Y. Ergogenic effects of 8 days of Sceletium tortuosum supplementation on mood, visual tracking, and reaction in recreationally trained men and women. J Strength Cond Res 2020;34(9):2476-81. PubMed
- Berry AR, Langley HN, Rogers RR, Benjamin CL, Williams TD, Ballmann CG. Effects of Zembrin (Sceletium tortuosum) supplementation on mood, soreness, and performance following unaccustomed resistance exercise: a pilot study. Nutraceuticals 2021;1(1):2-11. DOI
- ClinicalTrials.gov. NCT05471804: Psychological effects of 8 weeks supplementation with Sceletium tortuosum extract (Zembrin). ClinicalTrials.gov
- ClinicalTrials.gov. NCT07511088: A decentralized study utilizing non-invasive measures to evaluate a botanical supplement on episodes of mild acute stress and situational anxiety. ClinicalTrials.gov
- ClinicalTrials.gov. NCT01805518: Safety study of the effect of Sceletium tortuosum (as Zembrin) in aged normals. ClinicalTrials.gov
- de Jong M, van Niekerk S, Hamman J, van der Kooy F. Mesembryanthemum tortuosum and Zembrin: mixed evidence from in vivo animal and clinical studies on their antidepressant and anxiolytic effects. Planta Med 2026;92(8):806-823. PubMed
Frequently asked questions
How many human studies of kanna have been published?
We found seven published human studies of Sceletium tortuosum (Zembrin extract): Nell 2013, Terburg 2013, Chiu 2014, Dimpfel 2017, Reay 2020 (two experiments), Hoffman 2020 and Berry 2021. Together they enrolled fewer than 300 people, all healthy. Two more trials are registered but unpublished.
Have there been clinical trials of kanna in people with a medical condition?
We found none. Every published human study enrolled healthy volunteers, and a 2026 review noted that studies in clinically relevant populations are lacking [11].
Who funded the kanna trials?
Where a funding statement was available, funders included HG&H Pharmaceuticals (the company that developed Zembrin), its North American distributor PL Thomas, and a South African company, H.L. Hall and Sons [2][3]. Several authors are linked to HG&H. We could not read funding statements for every paper; the table marks those cases.
Did any trial test kanna drinks?
No. All seven studies gave capsules or tablets of a standardized extract, mostly 25 mg. Drinks use different extracts at different doses, so results do not transfer directly. See the dosage guide.
Is kanna proven to work?
No. The studies are small and short, they measured lab tasks and rating scales in healthy people, and results were mixed. Our benefits page summarizes what they can and cannot show.