Category: Herbs & Ingredients
Ashwagandha is one of the best-evidenced adaptogens, but also one of the most misapplied. Here is what it actually does for cortisol, sleep, anxiety and testosterone, what the clinical trials show, and who should not take it.
Category: Ingredient Intelligence | Reading time: ~18 min | Level: Intermediate
Every supplement goes through a moment. Echinacea had the 1990s. Fish oil was everywhere in the 2000s. Turmeric became prominent in the early 2010s. Ashwagandha has been prominent for the last decade and, unlike most supplements that peak and recede, it is still climbing. Sales continue to grow. The clinical literature continues to expand. And the questions people ask about it have become considerably more specific and considerably less credulous than they were five years ago.
That is a good development. Ashwagandha deserves scrutiny because its evidence base is robust enough to withstand it, which is more than can be said for most botanicals that achieve comparable commercial prominence. At the same time, the gap between what ashwagandha can legitimately do and what is claimed for it in mainstream marketing has widened alongside its popularity. Knowing the difference is what separates effective use from expensive disappointment.
This article covers ashwagandha in full: its history, its chemistry, how it works, what the clinical evidence actually shows, its limitations, and who is most likely to benefit from it. It is not a promotional piece. It is the kind of assessment a knowledgeable herbalist or clinical nutritionist would give to someone who said: "I keep hearing about ashwagandha. What does it actually do?"
Ashwagandha, botanical name Withania somnifera from the Solanaceae family, is a small shrub native to the Indian subcontinent, North Africa, and parts of the Mediterranean. The name comes from Sanskrit: ashva (horse) and gandha (smell), referring to the characteristic odour of the root and the traditional belief that consuming it would confer the strength and vigour of a horse. The species name somnifera is Latin for "sleep-inducing," though this is a partial description at best. Ashwagandha is calming rather than sedating in any clinical sense.
Within Ayurveda, it is classified as a rasayana, a category of tonifying medicines used to promote longevity, restore vitality after illness, and support cognitive and physical performance. Documented use dates back approximately 3,000 years, making it one of the longest continuously used medicinal plants in recorded history. That depth of traditional use is not just historically interesting. A plant with three millennia of documented continuous use in a sophisticated medical system has been subjected to a form of large-scale observational safety monitoring that no modern clinical trial can replicate.
Modern botanical medicine primarily uses the root. Leaf preparations exist, but the root contains the highest concentrations of the primary active compounds and forms the basis of virtually all clinical research.
Ashwagandha's pharmacological activity comes from several compound classes, of which the withanolides are the most studied and the most therapeutically significant.
Withanolides are steroidal lactones with a structural similarity to the steroid hormones that makes them uniquely capable of interacting with hormonal and stress-response systems. Withanolide A has been shown to modulate GABA-A receptor activity, contributing to the anxiolytic effect. Withaferin A demonstrates anti-inflammatory and apoptotic activity in preclinical research, though the latter is primarily of interest in oncology research rather than general wellness applications.
Alkaloids, including somniferine, somnine, withananine, and isopelletierine, contribute to the sedative and anti-anxiety effects through mechanisms that are less fully characterised but appear to involve cholinergic pathways.
Sitoindosides and glycowithanolides are bioactive glycosides with documented anti-stress and antioxidant activity in preclinical models and are the primary actives in some standardised preparations.
The quality of any ashwagandha product depends critically on withanolide content. Raw ashwagandha root powder typically contains 0.3 to 0.5% withanolides by weight. Standardised extracts concentrate this to 2.5 to 5% for root-only preparations. Two commercial extracts dominate the clinical literature:
KSM-66 (Ixoreal Biomed) is a full-spectrum, root-only extract standardised to a minimum of 5% withanolides, produced using a proprietary milk-based extraction process that preserves the broad compound profile. It has more published human RCTs behind it than any competing ashwagandha preparation.
Sensoril (Natreon Inc.) uses both root and leaf material and is standardised to a minimum of 10% withanolides plus 32% glycowithanolides. The leaf inclusion means a different compound profile, with higher concentrations of withaferin A. It appears particularly well-studied for cognitive and mood applications.
The choice between these two is not arbitrary. A clinician selecting ashwagandha for cortisol modulation should prioritise the KSM-66 literature. A clinician selecting it for cognitive support may weight the Sensoril evidence differently. Generic root powder with no standardisation, which constitutes the majority of the lowest-cost market, has no clinical trial basis for specific dosing.
Ashwagandha's clinical effects operate across several distinct biological systems. Most coverage treats them as a single undifferentiated action. They are not, and understanding which mechanism produces which effect is essential for deciding whether it fits a given situation.
HPA Axis Modulation
The most robustly evidenced mechanism is HPA axis modulation. Ashwagandha appears to act at the level of the hypothalamus, reducing corticotropin-releasing hormone (CRH) output, which is the initial signal that triggers the cortisol cascade. It also acts on the adrenal cortex itself, where its antioxidant activity may protect adrenocortical cells from oxidative damage associated with chronic ACTH stimulation. The net effect is a reduction in cortisol output under chronic stress conditions, with the bidirectional property of a true adaptogen. It reduces elevated cortisol without suppressing cortisol below healthy baseline levels.
This is not the same as cortisol suppression. Pharmaceutical corticosteroid inhibitors, used in conditions like Cushing's syndrome, block cortisol synthesis directly and can cause adrenal insufficiency. Ashwagandha's effect is regulatory rather than inhibitory and occurs specifically in chronically stressed populations.
GABAergic Activity
Withanolide A has been shown in preclinical research to modulate GABA-A receptor activity, specifically at the benzodiazepine binding site, contributing to the anxiolytic and calming effects reported clinically. Ashwagandha's interaction at this receptor appears milder and less direct than either benzodiazepines or passionflower's chrysin. The combination of HPA axis modulation and mild GABAergic activity likely explains why ashwagandha produces anxiolysis without the sedation associated with more potent GABAergic compounds.
Thyroid Modulation
This is an underappreciated effect with genuine clinical consequences. A 2018 randomised controlled trial by Sharma and colleagues in the Journal of Alternative and Complementary Medicine enrolled 50 adults with subclinical hypothyroidism and administered KSM-66 at 600 mg per day for eight weeks. The ashwagandha group showed statistically significant increases in both T3 (triiodothyronine) and T4 (thyroxine) levels alongside a reduction in TSH. This is a real therapeutic effect. It has positive implications for subclinical hypothyroidism and cautionary implications for anyone on levothyroxine or with hyperthyroidism.
Testosterone and Physical Performance
A 2015 randomised controlled trial by Wankhede and colleagues in the Journal of the International Society of Sports Nutrition enrolled 57 men undergoing resistance training and administered KSM-66 at 300 mg twice daily for eight weeks. The ashwagandha group showed significantly greater increases in muscle strength (bench press and leg extension), muscle recovery, and testosterone compared to placebo. A serum testosterone increase of approximately 15% was observed over the eight-week period.
The mechanism is almost certainly indirect. Sustained cortisol elevation suppresses testosterone via the pregnenolone steal, where pregnenolone, the shared precursor for both cortisol and the sex hormones, gets redirected toward cortisol synthesis. By modulating cortisol, ashwagandha appears to relieve this diversion and allow more substrate to reach testosterone synthesis pathways.
Ashwagandha has an unusually strong clinical trial base for a botanical. That is genuinely good news and also an invitation to read the studies carefully rather than cite them uncritically.
For cortisol and stress: The Chandrasekhar et al. (2012) RCT in the Indian Journal of Psychological Medicine remains the most cited. Sixty-four chronically stressed adults, KSM-66 at 300 mg twice daily versus placebo for 60 days, produced a 27.9% reduction in serum cortisol in the ashwagandha group versus 7.9% in placebo. Stress scores on validated questionnaires improved by 44% versus 5.5%. These are large effect sizes by any standard in this field.
A 2019 double-blind RCT by Lopresti and colleagues in Medicine enrolled 60 adults and administered KSM-66 at 240 mg per day for 60 days. Statistically significant reductions in cortisol, anxiety, and stress were observed at the lower dose, relevant for those concerned about higher-dose protocols.
A 2021 systematic review and meta-analysis in Medicine (Pratte et al.) pooled data from multiple RCTs and confirmed the cortisol-lowering and anxiolytic effects across studies, noting that withanolide standardisation variability between preparations limits cross-study comparison.
For sleep: A 2019 RCT by Langade and colleagues in Cureus administered KSM-66 at 300 mg twice daily for 10 weeks in 60 adults with insomnia complaints. The ashwagandha group showed significant improvements in sleep onset latency, total sleep time, and self-reported sleep quality. The likely mechanism is multifactorial: GABAergic activity plus cortisol reduction evening out the slope for melatonin rise in the late afternoon and evening.
For cognitive function: A 2017 study by Choudhary and colleagues in the Journal of Dietary Supplements administered Sensoril at 300 mg per day for eight weeks to 50 adults. Significant improvements appeared in immediate memory, general memory, executive function, sustained attention, and processing speed on neuropsychological testing.
For physical performance: The Wankhede et al. (2015) RCT demonstrated significant increases in strength and testosterone, with additional findings of suppressed exercise-induced muscle damage markers (creatine kinase and lactate dehydrogenase). A follow-up study by Ziegenfuss and colleagues in 2018 replicated the strength and recovery findings in trained men.
What the evidence does not show: Ashwagandha has not been convincingly demonstrated to produce rapid acute effects. Its mechanism is inherently time-dependent. Studies claiming meaningful cortisol changes in under 30 days should be interpreted with caution. Additionally, the evidence base is almost entirely in populations experiencing stress, sleep disruption, or training-related fatigue. Evidence for ashwagandha as a general wellness supplement in non-stressed individuals without specific physiological deficits is considerably thinner.
Ashwagandha has a well-documented safety profile at clinical doses. Adverse effects are typically mild and gastrointestinal: nausea and loose stools, occurring in a minority of users and generally resolving within the first week of use. The existing clinical trial data, primarily from 60-day and 90-day studies, does not raise significant safety concerns for most healthy adult populations.
Several contraindications are clinically important and non-negotiable:
Thyroid conditions: Given ashwagandha's documented stimulatory effect on T3 and T4, it is contraindicated in hyperthyroidism and Graves' disease. In individuals taking levothyroxine for hypothyroidism, it may enhance thyroid hormone levels in ways that interact with medication requirements. Prescriber awareness and monitoring is warranted before supplementation begins.
Autoimmune conditions: Ashwagandha is immunomodulatory. In conditions including Hashimoto's thyroiditis, lupus, rheumatoid arthritis, and multiple sclerosis, this immunomodulatory effect is unpredictable and use should only proceed with practitioner oversight.
Pregnancy: Preclinical data suggests potential abortifacient effects at higher doses. Modern clinical guidance is to avoid use during pregnancy unless under direct supervision of a practitioner experienced in both herbal and obstetric medicine.
Drug interactions: Significant interactions include sedative medications (additive CNS depression), immunosuppressants (opposing effects), thyroid medications (potentiation), and antidiabetic drugs (additive blood glucose lowering).
Dosing based on published clinical trial data:
Timing matters significantly. Given ashwagandha's calming-adaptive profile, morning-only dosing is less effective for individuals whose primary presentation is evening anxiety or sleep disruption. Split dosing, morning and evening, performs best across the clinical literature.
Sleep quality often improves within two to three weeks via its mild GABAergic effect, but the cortisol and stress-resilience changes measured in clinical trials emerge at around 60 days. Two weeks is not a fair assessment window.
For cortisol, stress and sleep, KSM-66 (a root-only extract standardised to ~5% withanolides) has the deepest RCT evidence. For cognition and mood, Sensoril (root + leaf, higher withanolide and withaferin A) has specific trial support. Generic non-standardised root powder has no clinical basis for specific dosing.
Clinical trials use 300 to 600 mg of a standardised extract per day, in one or two divided doses, for at least 60 days. Lower doses (120 to 240 mg) show effects in some studies; doses up to 1,000 mg appear safe but with diminishing returns.
Yes. The adaptogenic stress, sleep and anxiety effects apply regardless of sex; the testosterone findings are specific to men in training. The pregnancy and thyroid contraindications apply to everyone.
The longest well-designed RCTs run to about 90 days and traditional use spans millennia, but cycling roughly 8 to 12 weeks on and 4 weeks off is a pharmacologically conservative, common clinical recommendation.
This needs caution. Ashwagandha raised T3 and T4 in a 2018 RCT in subclinical hypothyroidism, a genuine therapeutic effect, but one that makes it contraindicated in hyperthyroidism/Graves' and potentially disruptive if you take levothyroxine. Speak to your prescriber and monitor TSH before starting.
There is no absolute contraindication, but ashwagandha has mild GABAergic and sedative-additive potential, so combining it with CNS depressants or sedatives can compound drowsiness. Clear it with the prescriber who manages your medication first.
No, that is the adaptogen distinction. It reduces *elevated* cortisol toward baseline rather than suppressing it, so it calms the stress response without the daytime flatness of a sedative. If you feel drowsy, shift more of the dose to the evening.
The effect is active modulation, it persists while you are taking it and tapers after you stop. It is not a permanent reset. The durable gains come from pairing it with the lifestyle changes that lowered the chronic stress load in the first place.