Ashwagandha is the UK's best-selling adaptogen — but what does it actually do? We break down the clinical evidence for stress, sleep, cortisol, and performance.
Ashwagandha (Withania somnifera) is a root herb native to India and North Africa that has been central to Ayurvedic medicine for over 3,000 years. Its Sanskrit name translates roughly as "smell of horse" — a reference both to its distinctive aroma and the traditional belief that it imparts the strength and vitality of a stallion.
It is now the UK's most widely sold adaptogen, found in everything from capsules to lattes to energy drinks. Sales grew 39% year-on-year in 2024 — driven partly by TikTok awareness and partly by a genuine body of clinical evidence that most other supplement ingredients can't match.
The primary bioactive compounds in ashwagandha are withanolides — steroidal lactones unique to the Withania genus. They exert effects primarily on the HPA (hypothalamic-pituitary-adrenal) axis — the hormonal system that governs your stress response — and on GABA receptors in the brain.
Quality matters significantly here. Look for standardised extracts — KSM-66 (full-spectrum root extract, 5% withanolides) and Sensoril (root and leaf, higher withanolide concentration) are the two most clinically studied and reliable forms.
This is where ashwagandha's evidence base is most robust. Multiple randomised controlled trials have demonstrated significant cortisol reduction:
The mechanism: withanolides inhibit CRH (corticotropin-releasing hormone) signalling, directly dampening the cascade that leads to cortisol secretion. They also have direct GABA receptor activity, reducing the neurological experience of anxiety.
Ashwagandha has significant evidence for improving sleep — not by sedation, but by reducing the physiological stress that prevents good sleep. Elevated evening cortisol is one of the most common causes of insomnia and poor sleep quality in otherwise healthy adults.
A 2019 randomised trial found ashwagandha root extract significantly improved sleep onset latency, sleep efficiency, and morning alertness after 10 weeks. The sleep quality improvements were most pronounced in participants with the highest baseline stress scores.
Beyond stress and sleep, ashwagandha has decent evidence for physical performance:
Several trials have found ashwagandha increases testosterone in men with normal or below-normal levels — primarily by reducing cortisol, which suppresses testosterone production. A 2019 trial found a 14.7% increase in testosterone in healthy men after 8 weeks of KSM-66 supplementation.
Effects on female hormonal health are less studied but suggest ashwagandha may help with symptoms of perimenopause and menopause by modulating cortisol and supporting adrenal function.
The vast majority of clinical research uses 300–600mg of KSM-66 or equivalent standardised extract per day. Products containing 100mg or less are likely underdosed. NECTA's formulas use 300mg — the minimum effective dose established across multiple trials — as part of a multi-ingredient blend.
Ashwagandha builds up over time. Most trials show peak effects at 6–8 weeks. You will not feel it dramatically on day one — but most people report a noticeable difference in how they respond to stressful situations after 2–4 weeks of consistent daily use.
Generally yes — it has an excellent safety profile across hundreds of clinical studies. Rare cases of liver injury have been reported at very high doses; use standard doses (under 600mg/day) from reputable suppliers. Avoid if: thyroid conditions (it modulates thyroid hormone), pregnancy, breastfeeding, immunosuppressant use, or nightshade allergy (ashwagandha is in the nightshade family).
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