Ashwagandha: Benefits, Side Effects, and the Right Dose — Evidence Review

Ashwagandha (Withania somnifera) has been used in Ayurvedic medicine for over 3,000 years — but in the last decade, it's earned a place in evidence-based supplementation through a strong body of randomised controlled trials.

This guide covers what it does, what it doesn't do, who should and shouldn't take it, and why the extract type changes everything.

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What ashwagandha actually does

Ashwagandha is an adaptogen — a compound that helps the body maintain homeostasis under stress. Its active compounds (withanolides) primarily work on the hypothalamic-pituitary-adrenal (HPA) axis, reducing cortisol output.

Proven benefits from RCTs:

Cortisol reduction: Multiple RCTs show KSM-66 ashwagandha (600mg/day for 8 weeks) reduces serum cortisol by 14–27% vs placebo. This is meaningful — elevated chronic cortisol impairs sleep, promotes fat storage, reduces testosterone, and accelerates ageing.

Sleep quality: A 2019 RCT found significant improvements in sleep onset latency, sleep efficiency, and mental alertness on waking vs placebo. The cortisol-lowering mechanism is likely responsible — evening cortisol is the main physiological barrier to sleep.

Anxiety and stress: Standardised scales (PSS, GAD-7, DASS) consistently show improved stress scores and reduced anxiety with KSM-66 supplementation.

Muscle strength and recovery: Several trials in resistance-trained men show ashwagandha supplementation increases bench press strength (by ~20kg over 8 weeks vs ~8kg in placebo) and reduces exercise-induced muscle damage markers.

Testosterone: Multiple RCTs show increases in testosterone in men (14–22% increase), primarily attributable to cortisol reduction and HPA normalisation rather than direct androgenic activity.

Thyroid: Early evidence suggests ashwagandha may increase thyroid hormone levels. Relevant for subclinical hypothyroidism but requires caution (see contraindications).

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The form problem

The research that proves these benefits was almost entirely conducted using:

- KSM-66 (full-spectrum root extract, standardised to 5% withanolides) — Ixoreal Biomed - Sensoril (root + leaf extract, standardised to 10% withanolides) — Natreon

Cheap ashwagandha root powder with undefined withanolide content is largely untested. If the label doesn't specify KSM-66, Sensoril, or a withanolide percentage, you can't verify what you're getting.

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Dosing

- KSM-66: 300–600mg daily, standardised to 5% withanolides. Most studies used 300mg twice daily or 600mg once daily in the evening. - Sensoril: 125–250mg daily (higher withanolide concentration). - Timing: Evening dosing aligns best with cortisol reduction for sleep benefit. Morning or split dosing for stress/performance. - Duration: Benefits build over 4–8 weeks. Most RCTs run 8 weeks for full effect.

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Side effects

Ashwagandha is well-tolerated in most people at standard doses. Reported side effects:

- Mild GI upset (most common — take with food) - Drowsiness (uncommon at recommended doses) - Rare: liver injury cases reported (mostly with high doses or poor-quality products)

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Contraindications — important

Thyroid disease: Ashwagandha can increase thyroid hormone levels. People with hyperthyroidism or on thyroid medication should avoid it or use under GP supervision.

Autoimmune conditions: As an immune modulator, use with caution in lupus, rheumatoid arthritis, MS.

Pregnancy: Avoid. There is historical use as an abortifacient in high doses.

Surgery: Stop 2 weeks before surgery — may interact with anaesthesia and affect blood pressure.

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Who benefits most

- People with chronic stress, high cortisol, or HPA axis dysregulation - Those with stress-related sleep disruption - Resistance-trained athletes seeking strength and recovery benefits - Men with signs of low testosterone secondary to chronic stress - Vegetarians/vegans who don't eat meat (some evidence of greater benefit in lower-baseline groups)

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Takeaway

Ashwagandha is one of the most clinically validated supplements for stress, sleep, and performance. The evidence is compelling — but only for standardised extracts (KSM-66 or Sensoril) at adequate doses. Cheap root powder is a different product. If the label doesn't specify the extract type and withanolide content, it's not worth buying.