Mood, Anxiety, and Cognitive Function

How Sceletium tortuosum, a South African succulent used for centuries, reduces anxiety and improves focus through dual serotonin and PDE4 mechanisms

Kanna — also known by its scientific name Sceletium tortuosum — is a small succulent plant native to South Africa's Cape region. For thousands of years, the indigenous San and Khoikhoi people fermented and chewed its leaves to lift mood, ease hunger and thirst on long journeys, reduce fear, and help with social bonding. Today it's attracting serious scientific attention because its active compounds work through two distinct brain pathways: serotonin reuptake inhibition (similar to antidepressants like SSRIs) and PDE4 inhibition (a mechanism associated with sharper focus and cognitive flexibility). [1] Human clinical trials show it measurably reduces anxiety responses in the brain and body, and improves executive function and cognitive set-switching — without the sedation of most calming herbs. [3][4] It is one of the more pharmacologically interesting mood herbs to emerge in recent decades.

How Kanna Works

Kanna's effects come primarily from a family of alkaloids called mesembrines, the most studied of which are mesembrine, mesembrenone, and mesembrenol. [1] These compounds act on two well-characterized neuropharmacological targets:

Serotonin reuptake inhibition: Mesembrine is a potent inhibitor of the serotonin transporter (SERT), the same mechanism targeted by SSRI antidepressants like fluoxetine and escitalopram. By slowing serotonin reuptake, it prolongs the availability of serotonin in synapses, supporting mood and reducing anxiety. What distinguishes mesembrine from synthetic SSRIs is its short duration of action — it works acutely (within hours), which is rarely seen with pharmaceutical SSRIs.

PDE4 inhibition: Mesembrenone inhibits phosphodiesterase type 4 (PDE4), an enzyme that breaks down cyclic AMP (cAMP) in brain cells. When cAMP levels rise, it activates a signaling cascade involving CREB (cAMP response element-binding protein), a transcription factor that promotes neuroplasticity, long-term memory formation, and cognitive flexibility. PDE4 inhibition is the same mechanism targeted by roflumilast (used in COPD) and is an active area of Alzheimer's research. [4] This means kanna does something most herbs do not: it may enhance both mood and cognition simultaneously through complementary pathways.

Anxiety and the Threat Response

A landmark 2013 brain imaging study used functional MRI to measure kanna's effects on the amygdala — the region most responsible for threat detection and anxiety. [3] Sixteen healthy adults were given a single 25 mg dose of Zembrin (a standardized kanna extract) or placebo in a crossover design. Under a perceptual load condition designed to reduce top-down control, the kanna group showed significantly attenuated amygdala reactivity to fearful faces compared to placebo. Connectivity between the amygdala and the hypothalamus (which drives physiological stress responses like elevated heart rate and cortisol release) was also reduced.

This is meaningful because the amygdala-hypothalamus circuit is the biological basis of the anxiety spiral — the rapid escalation from perceiving a threat to experiencing full physiological panic. Kanna appeared to dampen this circuit at its source, suggesting its anxiolytic effect is not simply sedation but a reduction in threat sensitivity itself.

Behaviorally, a 2020 placebo-controlled study found that 25 mg Zembrin significantly reduced subjective anxiety during a simulated public speaking task, and modulated heart rate during the stress challenge compared to placebo. [2]

Cognition and Executive Function

The PDE4 inhibition mechanism predicts cognitive benefits, and a small RCT tested this directly. [4] Twenty-one healthy adults aged 45–65 (not cognitively impaired) were randomized to 25 mg Zembrin or placebo daily for 3 weeks. The kanna group showed significantly better performance on tests of cognitive set flexibility (the ability to shift mental strategies) and executive function. The authors proposed this as a potential early intervention target for cognitive aging, given PDE4's role in the CREB pathway central to memory consolidation.

An 8-day study in 60 recreationally trained adults found that kanna supplementation improved performance on complex cognitive-motor tasks requiring visual tracking and reactive agility. [5] No significant changes in mood were observed in this athletic population, suggesting that the cognitive effects may be more reliable than mood effects in lower-stress healthy subjects.

Safety and Tolerance

The Nell et al. 2013 three-month safety trial (n=37, doses of 8 mg and 25 mg daily) found no significant adverse events at either dose compared to placebo. [2] Laboratory markers including liver enzymes, kidney function, blood counts, and vital signs were unchanged. Reported side effects were mild and similar across groups. There is no established dependence potential at therapeutic doses, though at very high doses kanna has historically been used for its mild intoxicating effects. The standardized extract used in clinical research (Zembrin) is formulated to provide predictable alkaloid levels while remaining well within the safe range.

Practical dosing: Most clinical studies used 25 mg daily of a standardized extract. Fermented whole leaf preparations used traditionally would require larger amounts. Sublingual absorption appears faster than oral capsules and may produce quicker acute effects.

Considerations: Because of its serotonin reuptake inhibitory activity, kanna should not be combined with prescription SSRIs, SNRIs, MAOIs, or other serotonergic medications due to theoretical risk of serotonin syndrome. Avoid during pregnancy. Not recommended alongside stimulants without medical supervision.

See our Ashwagandha page for a complementary adaptogen that reduces cortisol through a different (HPA axis) mechanism, and our L-Theanine page for another calming compound that supports cognitive performance without sedation.

Evidence Review

Pharmacological Foundation

Harvey et al. 2011 [1] established the pharmacological basis of kanna at the University of Strathclyde. The team tested a standardized extract (Zembrin) and three isolated alkaloids — mesembrine, mesembrenone, and mesembrenol — against human serotonin (SERT), dopamine (DAT), and norepinephrine transporters, along with PDE4A and PDE4B. Key findings:

Mesembrine was the most potent SERT inhibitor (IC50 not specified in abstract but described as potent and selective over DAT and NET)
Mesembrenone showed the strongest PDE4A inhibition (IC50 ~7.4 µM for PDE4A, 28.7 µM for PDE4B) with meaningful selectivity over other PDE subtypes
Mesembrenol showed intermediate activity at both targets
The alkaloid profile of the standardized extract explained its dual mechanism

The study established that the traditional preparation's pharmacology is consistent with its ethnobotanical use, and that the SERT and PDE4 mechanisms are not hypothetical but experimentally confirmed in human receptor assays.

RCT Safety and Tolerability: Nell et al. 2013

The Nell et al. 2013 trial [2] was a 12-week, randomized, double-blind, placebo-controlled, parallel-group safety study. Thirty-seven healthy adults received 8 mg Zembrin (n=12), 25 mg Zembrin (n=12), or placebo (n=13) once daily. Primary endpoints were clinical laboratory values (CBC, liver enzymes, creatinine, electrolytes), vital signs, ECG, and adverse events.

Results: No significant differences were found between groups on any laboratory measure. No serious adverse events occurred. The compound was described as well-tolerated at both doses across all three months. This study provides the foundation for dose selection in subsequent efficacy trials — it established that 25 mg daily was safe, well-tolerated, and appropriate for longer-term use in healthy adults.

Limitations: The trial was powered for safety, not efficacy, and had no primary mood or anxiety endpoints. The sample was small (n=37) and restricted to healthy adults without psychiatric diagnoses.

Neuroimaging Evidence: Terburg et al. 2013

The Terburg et al. 2013 pharmaco-fMRI study [3] is among the most rigorous mechanistic evidence for any herbal anxiolytic. Design details:

n=16 healthy adults, crossover design (each subject received kanna and placebo in randomized order)
Single dose: 25 mg Zembrin
Task: participants viewed fearful and neutral faces under high and low perceptual load conditions inside an MRI scanner
Primary outcome: BOLD signal in the amygdala; secondary outcome: functional connectivity between amygdala and hypothalamus

Under low perceptual load (when the amygdala can respond automatically to fearful faces without top-down suppression), kanna significantly attenuated amygdala BOLD response to threat stimuli compared to placebo. Amygdala-hypothalamus functional connectivity was also significantly reduced, specifically in circuits associated with triggering sympathetic stress responses.

The study is notable for testing a single dose and observing meaningful neural effects within hours — a time course consistent with the acute SERT and PDE4 inhibition documented by Harvey et al. The crossover design eliminates between-subject variability as a confound. Limitations include the small sample and the restriction to healthy, non-anxious participants; effects in clinical anxiety populations remain to be established.

Cognitive Enhancement: Chiu et al. 2014

The Chiu et al. 2014 proof-of-concept RCT [4] was published in Evidence-Based Complementary and Alternative Medicine. Design:

n=21 cognitively normal adults, mean age 54.6 years (range ~45–65)
25 mg Zembrin or placebo daily for 3 weeks, parallel-group, double-blind
Primary cognitive outcomes: CANTAB battery (Cambridge Neuropsychological Test Automated Battery), specifically tests of executive function, spatial working memory, and cognitive flexibility (set shifting)

Results: The Zembrin group showed statistically significant improvements over placebo in:

Cognitive set flexibility: p < 0.032
Executive function composite: p < 0.022

The authors interpreted these findings through the PDE4-cAMP-CREB pathway: PDE4 inhibition raises cAMP, which activates CREB, driving expression of genes involved in synaptic plasticity and working memory. The paper noted this pathway as a legitimate target for early intervention in age-related cognitive decline, given its role in both normal memory consolidation and Alzheimer's pathology.

Limitations: Very small sample (n=21), short duration (3 weeks), no clinical population, no long-term follow-up. The CANTAB battery is validated, but the effect sizes were not reported in ways that allow easy comparison to pharmaceutical standards. Replication with larger samples is needed.

Ergogenic and Cognitive-Motor Effects: Hoffman et al. 2020

Hoffman et al. 2020 [5] took a different approach, testing kanna in 60 recreationally trained healthy adults (mean age ~24) under conditions typical of athletic performance evaluation. Key findings:

8 days of 25 mg Zembrin improved performance on complex cognitive-motor tasks requiring reactive agility and visual tracking (p < 0.05) relative to placebo
No significant effects on mood scales in this population (though this group had low baseline stress)
No safety concerns noted

The study suggests kanna's cognitive benefits may extend to situations requiring rapid information processing and psychomotor coordination — not just anxiety reduction. The absence of mood effects in a healthy, low-stress athletic population is consistent with the hypothesis that kanna's mood benefits are most apparent when baseline anxiety or stress is elevated.

Evidence Strength Summary

The strongest evidence is for anxiety reduction (amygdala neuroimaging RCT) and cognitive flexibility (CANTAB RCT). Both are small-sample proof-of-concept trials in healthy volunteers — rigorous in design but not definitive. Safety evidence is good for 3-month use at 25 mg in healthy adults. The pharmacological mechanism is well-characterized from in vitro work. What is missing: large trials in clinical anxiety or depression populations, dose-ranging studies, longer-term efficacy data, and head-to-head comparisons with established interventions.

Overall confidence: moderate for acute anxiety modulation (well-mechanized, fMRI-confirmed, but small samples); moderate for cognitive enhancement (two positive small RCTs); high for short-term safety at 25 mg; low-moderate for antidepressant effects in clinical populations (no adequate trials).

References

Pharmacological actions of the South African medicinal and functional food plant Sceletium tortuosum and its principal alkaloidsHarvey AL, Young LC, Viljoen AM, Gericke NP. Journal of Ethnopharmacology, 2011. PubMed 21798331 →
A randomized, double-blind, parallel-group, placebo-controlled trial of Extract Sceletium tortuosum (Zembrin) in healthy adultsNell H, Siebert M, Chellan P, Gericke N. Journal of Alternative and Complementary Medicine, 2013. PubMed 23441963 →
Acute effects of Sceletium tortuosum (Zembrin), a dual 5-HT reuptake and PDE4 inhibitor, in the human amygdala and its connection to the hypothalamusTerburg D, Syal S, Rosenberger LA, Heany S, Phillips N, Gericke N, Stein DJ, van Honk J. Neuropsychopharmacology, 2013. PubMed 23903032 →
Proof-of-Concept Randomized Controlled Study of Cognition Effects of the Proprietary Extract Sceletium tortuosum (Zembrin) Targeting Phosphodiesterase-4 in Cognitively Healthy Subjects: Implications for Alzheimer's DementiaChiu S, Gericke N, Farina-Woodbury M, Badmaev V, Copen J, Husni M, Bhatt M, Gurguis G, Bhatt S, Cernovsky Z, Terpstra K, Kaushal SA, Bureau Y, Hou J, Milev R. Evidence-Based Complementary and Alternative Medicine, 2014. PubMed 25389443 →
Ergogenic effects of 8 days of Sceletium tortuosum supplementation on mood, visual tracking, and reaction in recreationally trained men and womenHoffman JR, Marcus I, Dubnov-Raz G, Gepner Y. Journal of Strength and Conditioning Research, 2020. PubMed 32740286 →