Adaptogenic Herbs for Stress: What the Research Shows

The word "adaptogen" has entered the mainstream. You will find it on supplement labels, in coffee blends, even in skincare. Like most terms that travel from research into marketing, it has picked up meanings along the way that the original science did not intend.

This guide goes back to what the research actually says. What does "adaptogen" mean in a scientific context? What mechanisms do clinical trials investigate? Among the herbs that carry the label, where is the evidence strong, where is it preliminary, and where is it thinner than the marketing suggests?

Three herbs receive the most research attention: ashwagandha, passionflower, and rhodiola rosea. Each interacts with the stress response through a different mechanism, and each has a different depth of evidence behind it.

What makes something an adaptogen?

The concept was formalised in the mid-20th century by Russian and Scandinavian researchers studying plants that appeared to increase the body's non-specific resistance to stress. An adaptogen, in the original definition, is a compound that helps the body adapt to stressors (physical, chemical, or biological) without causing significant side effects or disturbing normal function.

The important distinction is between an adaptogen and a sedative. A sedative dampens the nervous system directly. An adaptogen, in the research framework, is studied for supporting the body's own adaptive mechanisms: helping the stress-response system find its way back to balance rather than overriding it.

This framing matters because it sets the right expectation. Adaptogens are not fast-acting anxiolytics. The research on the most-studied adaptogens typically measures effects over weeks of consistent daily use, not hours after a single dose.

Ashwagandha: the withanolide mechanism

Ashwagandha (Withania somnifera) is the most-studied adaptogen in the current research literature. It has been used in Ayurvedic practice for centuries and is now the subject of multiple randomised, placebo-controlled human trials and recent meta-analyses.

How it interacts with the stress response

The active constituents are a group of steroidal lactones called withanolides. Mechanistic research suggests that withanolides modulate the hypothalamic-pituitary-adrenal (HPA) axis, the three-part loop that coordinates the body's stress response. When the brain registers a demand, the hypothalamus signals the pituitary, which signals the adrenal glands to release cortisol. This loop has a negative feedback mechanism: circulating cortisol tells the hypothalamus to ease off.

Withanolides appear to interact with this feedback machinery, influencing CRH (corticotropin-releasing hormone) signalling and glucocorticoid receptor sensitivity ¹. The studied effect is support for the feedback loop that brings cortisol back toward baseline after a stressor, not simple cortisol suppression.

What the clinical trials show

A 2025 systematic review and meta-analysis published in BJPsych Open analysed 15 studies with a combined 873 participants and found significant reductions in serum cortisol, perceived stress, and anxiety scores ². Doses across the included trials ranged from 125 to 600 mg daily, taken for 30 to 90 days.

Here is where the evidence gets genuinely nuanced. A separate 2025 meta-analysis in Nutrition and Health examined seven cortisol studies and six perceived-stress studies (488 participants combined) and found a statistically significant cortisol reduction (−1.16 ug/dL) but no significant impact on perceived stress ³.

This dissociation is worth sitting with. It suggests that ashwagandha may shift an objective biomarker (cortisol) without necessarily changing how stressed a person feels. The two do not always move together. This is a more honest reading of the evidence than a blanket claim about "stress relief."

Both meta-analyses reported that ashwagandha is generally well tolerated at the studied doses, with mild side effects. Long-term safety data beyond 90 days remain limited.

For a single, standardised ashwagandha extract of the kind used in these studies, Pure Encapsulations Ashwagandha provides a withanolide-standardised root extract at 500 mg per capsule, without binders, fillers, or unnecessary excipients.

Passionflower: the GABA pathway

While ashwagandha works on the HPA axis over weeks, passionflower (Passiflora incarnata) interacts with a different part of the stress picture: the GABA system.

GABA (gamma-aminobutyric acid) is the brain's principal inhibitory neurotransmitter, the molecule that quiets excitatory neural signalling and helps the nervous system downshift. When GABAergic tone is adequate, the transition from alert to at-ease is smoother. When it is insufficient, the subjective experience is often that "wired" quality that resists relaxation even when you are tired.

How passionflower interacts with GABA

Passionflower contains several flavonoid constituents, including chrysin, vitexin, isovitexin, and apigenin, that have been researched for their interactions with the GABAergic system. Laboratory studies report that these flavonoids bind to GABA-A receptors and inhibit GABA reuptake, increasing the effective availability of GABA at the synapse ⁴.

Apigenin has been studied for its binding activity at the central benzodiazepine site of the GABA-A receptor, the same receptor site that pharmaceutical anxiolytics target, though at a much lower affinity ⁴. This is the mechanistic basis for passionflower's presence in the calming-herb literature.

What the clinical evidence shows

The human trial evidence for passionflower is smaller than for ashwagandha but consistent in direction. A 2020 systematic review of nine clinical trials reported that the majority of studies found reduced measures of anxiety following passionflower administration ⁵. One trial compared passionflower directly with oxazepam (a benzodiazepine) for generalised anxiety and found comparable effectiveness, with passionflower producing fewer side effects such as dizziness, drowsiness, and confusion ⁶.

Passionflower has GRAS (generally recognised as safe) status and an overall favourable safety profile in the research. No adverse effects on memory or psychometric function have been reported in the trial literature ⁵.

The practical framing is that passionflower is positioned as calm without sedation: a way to support the GABAergic wind-down rather than force unconsciousness. This makes it relevant for daytime stress as well as evening use.

Metagenics NeuroCalm is a passionflower-centred formula that combines passionflower with zizyphus and magnolia, two other botanicals that have been researched for modulating GABAergic neurotransmission. The multi-herb approach is designed to support the GABA pathway from multiple angles, for the stress response and nervous tension.

Rhodiola rosea: the fatigue angle

Rhodiola rosea (golden root) occupies a different niche in the adaptogen research. Where ashwagandha is studied primarily for cortisol and the stress response, and passionflower for GABA and nervous tension, rhodiola has been investigated mainly for physical and mental fatigue.

The active markers are salidroside and rosavin. The proposed mechanisms involve modulation of stress-mediator expression and support of cellular energy metabolism under demanding conditions.

The evidence, honestly assessed

A systematic review identified 11 studies meeting inclusion criteria: 10 described as RCTs and one as a controlled clinical trial. Of six trials examining physical fatigue in healthy populations, two reported rhodiola to be effective. Of five RCTs evaluating mental fatigue, three found it effective ⁷.

These numbers are more equivocal than most rhodiola marketing suggests. The systematic review noted that all included studies exhibited either a high risk of bias or reporting flaws that make it difficult to assess their true validity ⁷. More recent reviews (2024) continue to describe rhodiola's effects as "generally minor and outcome-dependent," varying with dosing schedule, standardisation, and the specific task measured.

There are signals of benefit, particularly for endurance-related outcomes and mental fatigue under acute stress. But the evidence base is limited and the study quality is uneven. A reader who values evidence over marketing should weight rhodiola accordingly: as an interesting but preliminary option.

We do not currently carry a rhodiola product and include it here for completeness and honest comparison rather than as a recommendation.

Choosing an adaptogen: different herbs for different needs

The three herbs above address different parts of the stress picture. The right choice depends on what you are trying to support.

Ashwagandha has the strongest evidence base for supporting a healthy cortisol rhythm and HPA-axis response over weeks of daily use. The meta-analysis data on cortisol reduction are consistent, and the mechanism is well-characterised.

Passionflower formulas are better suited for the moment of winding down: the evening transition from aroused to at-ease, or daytime nervous tension that you want to modulate without sedation. The GABA-pathway mechanism operates on a different timescale from HPA-axis adaptogens.

Rhodiola sits in the fatigue and acute-stress performance space. If the evidence matures, it may earn a stronger position. For now, the data support cautious interest rather than confident recommendation.

These categories are not mutually exclusive. Ashwagandha and a passionflower formula address different pathways and could be used together by someone who wants to support both the daily cortisol rhythm and the evening wind-down. For a deeper look at how these pathways connect, including the HPA axis, the diurnal cortisol curve, and the GABA-mediated brake that helps you downshift at night, our Stress and Sleep Guide covers the full picture.

What adaptogens do not replace

It is worth being explicit about the limits of what any supplement can do in the context of stress.

The interventions with the largest evidence base for stress resilience are not pills. They are consistent sleep timing, regular physical movement, morning light exposure, caffeine management, and genuine recovery: the behaviours that set the conditions for a healthy stress-response system. Adaptogens are studied as one input among these foundations, not as a substitute for them.

If stress has crossed the line into something that is affecting your ability to function (persistent sleep disruption, anxiety that does not respond to lifestyle changes, or symptoms that concern you), a qualified healthcare professional is the right next step.

The research on adaptogens is real and growing. The most useful way to think about these herbs is as a considered addition to a solid foundation.

This article describes findings from published research for general educational purposes. It is not medical advice, and nothing here is intended to diagnose, treat, cure, or prevent any disease. If you take prescription medication or have a health condition, consult a qualified healthcare professional before adding a supplement.

References

¹ Withanolide HPA-axis mechanism: reviewed in NIH ODS Ashwagandha Health Professional Fact Sheet; Singh, N. et al. (2011). An overview on Ashwagandha. African Journal of Traditional, Complementary and Alternative Medicines, 8(5S).

² Bachour, G. et al. (2025). Effects of ashwagandha supplements on cortisol, stress, and anxiety levels in adults: a systematic review and meta-analysis. BJPsych Open. PMC12242034. 15 studies, 873 participants.

³ Albalawi, A. (2025). Dual impact of Ashwagandha: significant cortisol reduction but no effects on perceived stress. Nutrition and Health (SAGE). DOI: 10.1177/02601060251363647. 7 cortisol studies, 6 perceived-stress studies, 488 participants.

⁴ Passionflower GABA mechanism: Appel, K. et al. (2011). Modulation of the gamma-aminobutyric acid (GABA) system by Passiflora incarnata L. Phytotherapy Research, 25(6), 838-843. PMID: 21089181.

⁵ Janda, K. et al. (2020). Passiflora incarnata in neuropsychiatric disorders: a systematic review. Nutrients, 12(12), 3894. PMC7766837. 9 clinical trials reviewed.

⁶ Akhondzadeh, S. et al. (2001). Passionflower in the treatment of generalized anxiety: a pilot double-blind randomized controlled trial with oxazepam. Journal of Clinical Pharmacy and Therapeutics, 26(5), 363-367.

⁷ Ishaque, S. et al. (2012). Rhodiola rosea for physical and mental fatigue: a systematic review. BMC Complementary and Alternative Medicine, 12, 70. PMC3541197.