Ashwagandha (Withania somnifera, also known as Indian ginseng and winter cherry) is one of the most important herbs in Ayurvedic medicine — classified as a rasayana, a herb that promotes physical and mental vitality and longevity. Its primary active constituents are the withanolides, a group of steroidal lactones that have broad effects on the hypothalamic-pituitary-adrenal (HPA) axis, on the GABAergic system, and on the immune system. The most clinically relevant application of ashwagandha in modern practice is its ability to modulate cortisol and enhance stress resilience — reducing the physiological and behavioural consequences of chronic stress without the sedation or dependence liability of pharmaceutical anxiolytics.
The HPA Axis and Cortisol Dynamics
The HPA axis is the body’s primary stress response system: the hypothalamus secretes corticotropin-releasing hormone (CRH), which stimulates the anterior pituitary to release adrenocorticotropic hormone (ACTH), which stimulates the adrenal cortex to secrete cortisol. When cortisol is chronically elevated — as it is in chronic stress, sleep deprivation, and Cushing syndrome — it produces a characteristic pattern of central adiposity, muscle wasting, immune suppression, insulin resistance, and cognitive impairment. Ashwagandha reduces cortisol at the level of the hypothalamus (reducing CRH secretion) and possibly at the adrenal level (reducing the cortisol response to ACTH stimulation).
A double-blind RCT in 64 adults with chronic stress found that ashwagandha root extract at 300mg twice daily significantly reduced cortisol levels (from a mean of 19.3 to 11.4 mcg/dL), improved stress scores on the Perceived Stress Scale (from 26.1 to 17.3), and reduced symptoms of anxiety (Hamilton Anxiety Rating Scale from 34.8 to 21.2) compared to placebo after 8 weeks of treatment. These are clinically meaningful reductions — comparable to the effects of pharmaceutical anxiolytics but without the dependence, tolerance, or withdrawal liability.
Ashwagandha for Physical Performance and Recovery
Ashwagandha has been studied extensively in athletes for its effects on strength, endurance, and recovery. A double-blind RCT in 57 young male subjects with no prior resistance training found that ashwagandha root extract at 300mg twice daily for 8 weeks significantly increased muscle strength (bench press: +20.5kg vs +8.2kg in placebo), muscle size, and testosterone levels, while reducing exercise-induced muscle damage (measured by CK levels) and improving recovery time between training sessions. This anabolic effect on muscle is consistent with ashwagandha cortisol modulation — elevated cortisol is catabolic to muscle and directly opposes the anabolic effects of resistance training.
Ashwagandha for Thyroid Function and Metabolism
Emerging evidence suggests that ashwagandha may support thyroid function in people with subclinical hypothyroidism. A double-blind RCT in 50 adults with elevated TSH and normal T3/T4 (subclinical hypothyroidism) found that ashwagandha at 600mg daily for 8 weeks normalised TSH (from a mean of 9.2 to 4.3 mcIU/mL), increased T3 (+18%), and increased T4 (+7%) compared to placebo. This is consistent with the traditional Ayurvedic use of ashwagandha for thyroid support and suggests it may be a useful adjunctive intervention for subclinical hypothyroidism, particularly in people with elevated stress hormones contributing to thyroid dysfunction.
Practical Dosing and Quality
The evidence-based dose for stress reduction, cortisol modulation, and physical performance is 300-600mg daily of KSM-66 ashwagandha root extract (the most clinically studied form, standardised to >5% withanolides) or 600-1200mg daily ofSensoril ashwagandha root and leaf extract (standardised to >10% withanolides, more studied for cognitive and anxiety applications). Side effects are rare at these doses but include mild GI discomfort, drowsiness, and, uncommonly, thyroid hormone elevation — people with thyroid disease should use ashwagandha under medical supervision.
Side effects from ashwagandha are uncommon at standard doses (300-600mg daily of KSM-66 extract), but very high doses above 1,200mg daily of root extract have been associated with mild digestive discomfort in some people. Anyone with a thyroid condition should use ashwagandha with caution, as it can occasionally stimulate thyroid activity — worth discussing with your endocrinologist if you are on thyroid medication. Pregnant women should avoid ashwagandha entirely as a precautionary measure, since there is insufficient safety data in pregnancy. Most adults can use it safely for up to 12 weeks before reassessing.
The KSM-66 extract of ashwagandha is the most clinically studied form and the one used in most published randomized controlled trials. It is a root-only extract, standardised to at least 5% withanolides, and the research evidence supports its use for reducing perceived stress scores (measured by validated questionnaires like the Perceived Stress Scale), improving sleep quality, and modestly enhancing physical performance and recovery in athletes. The effects on cortisol are real but modest — typically a 20-30% reduction in morning cortisol after 8 weeks of consistent use, which is clinically meaningful if your cortisol is elevated but not dramatic if it is already normal. The more noticeable effect for most users is improved sleep onset — falling asleep faster and waking less during the night — which may be secondary to the cortisol-lowering effect rather than a direct sedative action.
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