**Ashwagandha (*Withania somnifera*) is the most clinically studied herbal adaptogen.** A 2019 double-blind trial of 60 chronically stressed adults showed KSM-66 extract at 600 mg/day cut serum cortisol by 27.9% over 60 days versus 7.9% for placebo (Salve et al., *Cureus*). That's a large effect for a plant with a 3,000-year safety record.
Popular coverage tends to over-promise ("cures anxiety") or dismiss it ("just another supplement fad"). The published trials say something more precise: ashwagandha reliably reduces stress markers, sleep latency, and generalised anxiety in adults with sub-clinical distress, over weeks. It's not a benzodiazepine substitute and it's not a placebo.
What ashwagandha actually is
The medicinal part is the *root* — not the leaf or the berry. Traditional Ayurvedic preparations use dried root powder in warm milk; modern trials almost always use standardised extracts. This matters because withanolide content (the active compound family) varies 4-fold between root and leaf, and root-only extracts have the strongest evidence base.
Two extracts dominate the clinical literature: KSM-66 (12 studies, ~1,200 participants, root-only, standardised to 5% withanolides) and Shoden (fewer trials but higher withanolide concentration at 35%). Generic powders and cheap capsules rarely match the standardisation used in trials — if the label just says "ashwagandha extract 500 mg" with no withanolide percentage, treat it as unknown-quality.
The four evidence-supported uses
Chronic stress and cortisol. Multiple RCTs show 300-600 mg/day of KSM-66 lowers serum and salivary cortisol by 20-30% over 6-8 weeks in stressed adults. Perceived stress scores drop in parallel. Effect size is comparable to a beginner mindfulness practice but requires no behavioural change from the user — that combination is unusual.
Sleep. Chronic stress fragments sleep architecture. Ashwagandha's cortisol-lowering effect translates into faster sleep onset (mean reduction ~13 minutes) and improved sleep efficiency. Best-studied in older adults (65-80) at 600 mg/day of KSM-66 for 12 weeks. Different mechanism than valerian or chamomile — those work acutely; ashwagandha works over weeks.
Athletic performance. Trials in recreationally active men show 300-600 mg/day increases VO₂max, muscle strength, and recovery markers over 8 weeks. Effect is modest in absolute terms but consistent across studies. Adaptogenic modulation of HPA-axis stress from training likely explains it — not direct muscle-building.
Testosterone in stressed men. Small trials show ~15% increase in total testosterone in men with sub-clinical stress or infertility over 90 days. This is the finding most commonly over-marketed. It's real but modest, and doesn't apply to men with normal baseline testosterone or to women (in whom ashwagandha does *not* raise testosterone despite bro-science claims).
How to dose it
Standard clinical dose: 300-600 mg/day of a root extract standardised to at least 5% withanolides (KSM-66 or equivalent), taken with food, ideally split between morning and evening or taken once at night if sleep is the target.
Duration matters. Below 4 weeks, effects are marginal in most trials. The dose-response curve plateaus around 600 mg/day for KSM-66 — going higher doesn't help and increases GI side-effects.
Cycling isn't strictly required but many practitioners recommend 8-12 weeks on, 2-4 weeks off, to reassess. Tolerance doesn't seem to develop in the studied timeframes.
Who shouldn't take it
Thyroid patients. Ashwagandha reliably raises T3 and T4 in hypothyroid patients (this is often a *desired* effect, but for hyperthyroid patients or those on stable levothyroxine it can destabilise TSH). Check thyroid labs at 6-8 weeks if you're on medication.
Autoimmune disease. Ashwagandha is mildly immunostimulant. For most people this is neutral or positive, but rheumatoid arthritis, lupus, MS, and Hashimoto's patients should discuss with a clinician before use.
Pregnancy and lactation. Ashwagandha has traditional use as an abortifacient in high doses in Ayurveda. Modern trials exclude pregnant participants and there's no reliable safety data. Defer use.
Sedative medications. Ashwagandha can potentiate benzodiazepines, barbiturates, and Z-drugs. Don't stack.
Recent surgery. Immunomodulating and mildly sedating — stop 2 weeks pre-op.
Common side effects
At clinical doses ashwagandha is well-tolerated. The most-reported side effects are mild GI upset (5-10% of trial participants) and drowsiness. Rare case reports of liver enzyme elevation exist — mostly with high-dose (>1200 mg/day) unregulated products.
The characteristic root smell (some people describe it as musty horse or barnyard — the Sanskrit name translates as "smell of horse") is normal for authentic root powder. Odourless capsules aren't automatically fake, but the smell is a mild authenticity marker.
What to look for in a product
- Root-only. Full-spectrum "whole plant" products dilute the withanolide-rich root with lower-potency leaf.
- Standardised withanolide content stated. KSM-66 is 5%, Shoden is 35%, both are well-studied. Anything lower than 2.5% is unlikely to match trial doses.
- Third-party tested. Ashwagandha is imported almost entirely from India; heavy-metal contamination is not rare. USP verification, NSF Certified for Sport, or Consumer Lab tested are meaningful marks.
- Reasonable price. Standardised extracts cost €0.30-0.60 per daily dose at retail. €0.10 doses are almost always weak generic powder.
Common questions
Ashwagandha or [rhodiola](/plants/rhodiola-rosea)? Different profiles. Rhodiola is more stimulating and better for acute mental performance (exam, presentation, tough week). Ashwagandha is more sedating and better for chronic stress and sleep. Not typically combined.
Ashwagandha or [chamomile](/plants/matricaria-chamomilla) for anxiety? Chamomile works within an hour for acute anxiety; ashwagandha requires weeks but changes baseline. Different tools for different jobs — you can use both.
How fast does it work? Sleep and stress benefits typically emerge 2-4 weeks in. Full effect at 8-12 weeks. If nothing after 8 weeks at a proper dose, it's unlikely to work for you.
Can I take it with SSRIs? No published interactions but shared serotonergic activity means theoretical risk of serotonin syndrome at high doses. Talk to your prescriber.
Further reading
- Adaptogens 101 — the broader class
- Sleep herbs that actually work — comparison of options
- Plant: Withania somnifera — full profile with citations
- Goal: stress — other options for the same problem
- Glossary: adaptogen — what the term actually means
This article summarises published clinical trials as of 2026 and is not medical advice. Individual response varies. Discuss ashwagandha use with a clinician if you take prescription medication or have a chronic condition — especially thyroid, autoimmune, or psychiatric.