Ashwagandha

Ashwagandha (Withania somnifera) is a root used in Ayurvedic medicine and now one of the most widely sold "adaptogen" supplements in the UK, marketed for stress, anxiety, sleep, energy and testosterone. Some of these uses have reasonable supporting evidence; others are weaker than the marketing suggests. Two safety issues - liver injury and thyroid effects - are absent from most consumer guides to ashwagandha, despite being more clinically significant than most of the marketed benefits.

Despite the nickname "Indian ginseng", ashwagandha is not a ginseng (Panax species) - it's an unrelated plant in the nightshade family. The name reflects a marketing comparison, not a botanical relationship.

Does it work? The evidence by claimed use

Lowering cortisol

Evidence: Moderate.

Several meta-analyses have found a statistically significant reduction in cortisol (the stress hormone) after around 8 weeks of supplementation, with effect sizes fairly consistent across different pooled analyses - the most consistently replicated finding for ashwagandha.

Feeling less stressed

Evidence: Limited.

Lower cortisol does not necessarily mean people feel less stressed. Trial evidence for self-reported stress is less consistent than for the cortisol biomarker: one 2025 meta-analysis found a significant cortisol reduction but no effect on standard stress questionnaires, while a separate meta-analysis found a significant improvement in perceived stress scores. The two bodies of evidence do not agree, which matters for anyone taking ashwagandha specifically to feel calmer rather than to change a lab value.

Anxiety

Evidence: Limited.

Several meta-analyses report large improvements on clinical anxiety rating scales, but the researchers conducting these reviews rate the overall certainty of the evidence as low, mainly because there are still few trials, considerable variation in design, and weaknesses in how participants were blinded to treatment.

Sleep

Evidence: Limited.

A 2021 systematic review found a modest improvement in sleep quality, more consistent in people with existing sleep problems such as insomnia than in generally healthy sleepers.

Memory, attention and focus

Evidence: Moderate.

A 2024/2025 meta-analysis pooling 20 studies (1,249 participants) found improvements in memory, attention/processing speed and executive function, with moderate rather than large effect sizes. The researchers describe the overall area as "controversial" despite the pooled results, reflecting the early stage of this evidence base compared with ashwagandha's better-known "stress relief" reputation.

Exercise performance and strength

Evidence: Moderate.

Two meta-analyses found a consistent improvement in VO2max (aerobic capacity) of roughly 3-4 ml/min/kg compared with placebo; evidence for strength and recovery is more limited, with improvements reported at 500-1,250mg/day over 8-12 weeks. Nearly all trials in these reviews were conducted in India, which narrows how confidently the results generalise to other populations and training contexts.

Testosterone and male fertility

Evidence: Limited.

The strongest trial results - increases in sperm count, motility and semen volume - come from studies in men with low sperm count (oligospermia) or stress-related fertility issues, not from healthy men generally. One trial in healthy, stressed men aged 21-45 found a roughly 15% increase in testosterone over 8 weeks at 600mg/day. The mechanism may be partly indirect: lowering cortisol could ease stress-related suppression of reproductive hormones, rather than ashwagandha directly increasing testosterone production. Outside a diagnosed fertility issue, "ashwagandha boosts testosterone" remains an area of early-stage research rather than an established effect.

Thyroid function

Evidence: Moderate.

A placebo-controlled trial in 50 adults with subclinical hypothyroidism (mildly raised TSH, not full hypothyroidism or people on thyroid medication) found that 600mg/day of ashwagandha root extract for 8 weeks normalised TSH in 79% of the treatment group, compared with 11% on placebo. This is a single, unreplicated trial in a specific population - not evidence that ashwagandha is a general thyroid tonic — and is one side of a safety consideration covered below.

Lamberts Ashwagandha Complex Solgar Ashwagandha Root Extract Dr Vegan Ashwagandha KSM-66 Terranova Ashwagandha 250mg

Safety

Liver injury

Published case reports describe liver injury linked to ashwagandha, usually appearing 2 to 12 weeks after starting it, with jaundice and itching as the most common symptoms. The NIH's LiverTox database classifies ashwagandha as a probable cause of clinically apparent liver injury. Most reported cases resolve within a few months of stopping, but rare fatal cases and cases requiring liver transplantation have been described, particularly in people with pre-existing liver disease. If you develop yellowing of the skin or eyes, dark urine, unusual itching, or persistent nausea while taking ashwagandha, stop and seek medical advice, and mention ashwagandha specifically - it's easily missed as a cause. Avoid ashwagandha if you have existing liver disease, or take it only under medical supervision.

Pharmacist comment

"Herbal supplements are still medicines from a clinical point of view, even though they're bought without a prescription. If jaundice, dark urine, itching or unusual nausea develop while taking ashwagandha, mention it specifically when you seek advice, alongside any prescribed medicines - it gives whoever assesses you the complete picture needed to identify the cause."

-Alessandro Grenci, Superintendent Pharmacist at Medino

Thyroid effects

Ashwagandha's measurable activity on thyroid hormone levels (see the subclinical hypothyroidism trial above) is a risk for people with an overactive thyroid (hyperthyroidism) or Graves' disease - published case reports describe thyrotoxicosis, in one case with a dangerous fast heart rhythm, attributed to ashwagandha. Avoid ashwagandha if you have hyperthyroidism. If you take thyroid medication (levothyroxine) for hypothyroidism, check with a GP or pharmacist before starting ashwagandha, since it could change how much medication you need.

Pregnancy and breastfeeding

Ashwagandha is commonly described as unsafe in pregnancy due to a risk of miscarriage. The evidence behind that specific claim is weaker than it's usually presented, and expert reassessment has questioned it. This does not mean ashwagandha is safe in pregnancy - reliable safety data is simply lacking either way. The practical advice remains precautionary: **avoid ashwagandha in pregnancy and while breastfeeding.**

Other interactions

  • Sedatives (benzodiazepines, "Z-drug" sleeping tablets like zolpidem): combining with ashwagandha may increase drowsiness.
  • Diabetes medication: ashwagandha may lower blood glucose; combined with glucose-lowering medication, this raises the risk of levels dropping too low.
  • Blood pressure medication: ashwagandha may lower blood pressure, so combining with BP-lowering medication needs a pharmacist check.
  • Immunosuppressants (e.g. after an organ transplant, or for an autoimmune condition - ciclosporin, tacrolimus, corticosteroids): ashwagandha may have immune-stimulating activity that could work against these medicines. Do not take ashwagandha alongside immunosuppressant treatment without specialist advice.

Most short-term (8-12 week) trials in otherwise healthy adults report only mild side effects (typically digestive upset or drowsiness). Data beyond a few months of continuous use is limited, so long-term, open-ended daily use carries more uncertainty than the short courses most trials studied.

Dose and forms

The two most-studied branded extracts use different parts of the plant, are standardised differently, and are studied at different doses:

  • Root-only extracts (e.g. KSM-66), standardised to a minimum of 5% withanolides (the plant's active compounds), are typically studied at 300-600mg/day.
  • Root-and-leaf extracts (e.g. Sensoril), standardised to around 10% withanolides, are typically studied at a lower 125-250mg/day, reflecting the higher withanolide concentration per gram.

Comparing two products by total milligrams alone, without checking which extract and standardisation is used, compares different things. If a label doesn't state the withanolide percentage or the branded extract used, the dose can't be meaningfully judged against the research. R.Y.M. KSM-66 Ashwagandha 600mg is a root-only extract at the upper end of the studied dose range, for reference.

Most positive trial results come from 8-12 week courses. There is limited evidence either way on continuous, indefinite daily use beyond that.

Is ashwagandha worth taking?

The strongest evidence is for reductions in cortisol, with more limited evidence for improvements in memory, attention and aerobic performance. Evidence for perceived stress and anxiety is less certain than the cortisol data alone would suggest, and "testosterone booster" claims are only well supported in men with a diagnosed fertility issue, not as a general effect.

The two safety issues marketing rarely mentions - liver injury and thyroid effects - are more relevant to most people's decision than any of the marketed benefits. Anyone with liver disease or an overactive thyroid should avoid ashwagandha, and anyone on thyroid medication, immunosuppressants, or medication for diabetes or blood pressure should check with a pharmacist first. Ashwagandha is not recommended in pregnancy or while breastfeeding.

Sources

  • Rhodiola rosea - another adaptogen for stress and fatigue, with a more limited evidence base.
  • L-Theanine - calms via brain neurotransmitters rather than cortisol, for anxious feelings specifically.
  • Valerian - a longer, more consistent evidence trail for sleep onset.
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