KSM-66 ashwagandha at 600mg/day reduced cortisol by 22–27% in multiple placebo-controlled RCTs. A 2025 12-month safety study in 191 adults found no serious adverse events. Here's what the evidence says — and the side effects you need to know before taking it.
Ashwagandha has been used in Ayurvedic medicine for over 3,000 years. In the past decade it's accumulated a clinical trial record that few adaptogens can match. The KSM-66 extract specifically — a root-only, water-extracted, standardised form — has been studied in over 20 randomised controlled trials covering stress, cortisol, sleep, testosterone, cognition, and athletic performance. The quality of that evidence base is genuine, though context matters.
The Clinical Evidence
The landmark 2012 study (Chandrasekhar et al., PMC3573577) randomised 64 adults with chronic stress to KSM-66 300mg twice daily or placebo for 60 days. The ashwagandha group showed significant reductions across all stress scales (p<0.0001) and serum cortisol was substantially reduced (p=0.0006). This remains the most-cited ashwagandha study and the one that established KSM-66 as the reference standard.
A later 2019 dose-finding study compared 250mg/day, 600mg/day, and placebo over eight weeks. The 600mg group showed statistically significant reductions in anxiety (Hamilton Anxiety Scale), stress (Perceived Stress Scale), and cortisol. The 250mg group showed reduced cortisol but didn't reach significance on anxiety metrics. The takeaway: 600mg/day is the clinically supported dose.
The 2025 12-month safety study in 191 adults aged 18–65 is the most clinically meaningful piece of recent evidence. KSM-66 at 600mg/day for 12 months showed no significant changes in liver, kidney, or thyroid function. Cortisol modestly declined. Testosterone increased in both men and women. Quality of life improved significantly, with 68.7% of participants showing overall clinical improvement — particularly those over 50.
Benefits That Are Well-Supported
Stress and cortisol reduction: The most consistent finding across trials. Multiple RCTs show 20–30% reductions in serum cortisol with regular use, alongside improvements in perceived stress and anxiety. The World Federation of Societies of Biological Psychiatry provisionally recommends ashwagandha for generalised anxiety disorder.
Sleep quality: A 2019 RCT in 80 adults (half with insomnia) found KSM-66 improved sleep onset latency, total sleep time, sleep efficiency, and morning alertness versus placebo. Benefits were more prominent in the insomnia group. The NIH Office of Dietary Supplements notes the evidence for sleep is "small but significant" across five pooled studies.
Testosterone: The 2025 12-month study confirmed testosterone increases in both sexes. Earlier trials showed 15–17% increases in testosterone in men with low-normal levels, alongside improvements in sperm quality. The mechanism appears to involve hypothalamic-pituitary-gonadal axis regulation via cortisol reduction rather than direct testosterone synthesis.
Athletic performance: Multiple studies show improvements in VO₂ max, muscle strength, and recovery. A 2015 study in healthy adults found significant increases in muscle mass and strength alongside reductions in exercise-induced muscle damage markers at 300mg twice daily.
Side Effects & Who Should Avoid It
Common mild side effects (9.4% incidence in the 12-month study, comparable to placebo): loose stools, nausea, drowsiness. Usually mild and transient, particularly at initiation. Taking with food reduces GI effects. The sedative effect means taking it at night or with dinner is preferable if drowsiness is an issue.
Serious but rare: There are case reports of liver injury associated with ashwagandha supplements — the 2025 12-month study found a slight increase in ALT (liver enzyme) that was not clinically significant, but liver toxicity reports in real-world use prompt caution in anyone with liver disease or on hepatotoxic medications.
Who Should Avoid or Use With Caution
Thyroid conditions: Ashwagandha can alter thyroid hormone levels — both up and down depending on individual response. Anyone with hypothyroidism, hyperthyroidism, or taking thyroid medication should consult a doctor before use. Pregnancy and breastfeeding: Avoid entirely — ashwagandha has traditionally been used as an abortifacient in high doses. Autoimmune conditions: As an immune stimulant, it could potentially worsen conditions like lupus, MS, or rheumatoid arthritis. Medications: Consult a doctor if taking sedatives, anxiolytics, immunosuppressants, or diabetes or blood pressure medications.
Dosing & Cycling
600mg/day is the evidence-supported dose — typically 300mg twice daily with food. KSM-66 is the most studied extract; look for products standardised to at least 5% withanolides. Takes 2–4 weeks to notice stress effects; full benefits build over 8–12 weeks of consistent use.
On cycling: ashwagandha has a short half-life and the body can develop tolerance to its cortisol-modulating effects. Taking it 5 days on/2 days off, or taking a 1–2 week break every 6–8 weeks, preserves sensitivity. This isn't based on strong clinical data but is a reasonable precautionary approach.
Key Takeaways
- KSM-66 at 600mg/day has consistent RCT evidence for cortisol reduction (20–30%), stress and anxiety relief, and sleep improvement. The evidence base is genuine and well-controlled.
- The 2025 12-month safety study (191 adults) found no serious adverse events and confirms it's safe for extended use — though longer-term data beyond 12 months is still limited.
- 600mg/day (300mg twice daily) is the clinically supported dose. 250mg may reduce cortisol but doesn't reach significance on anxiety metrics in dose-comparison studies.
- Avoid in pregnancy (potential abortifacient in high doses), thyroid conditions, and autoimmune disease without medical supervision.
- Cycle periodically — 5 days on/2 days off or 6 weeks on/1–2 weeks off — to maintain cortisol sensitivity.
- Only buy KSM-66 standardised extract from third-party tested brands (Momentous, Thorne). Label accuracy and withanolide content vary enormously across cheap products.