Adaptogens: What Ashwagandha and Rhodiola Can and Can't Do

What “adaptogen” means

Adaptogens are a category of herbs traditionally said to help the body resist and adapt to stress. It is a useful umbrella term, not a precise mechanism, so as always the question is what the trials show for each specific herb.

The label came out of Soviet pharmacology in the mid-twentieth century and has never had a regulatory definition. In practice it now covers a dozen or so plants - ashwagandha, rhodiola, holy basil (tulsi), ginseng, eleuthero, schisandra - that share a marketing story more than a biology. Two of them, ashwagandha and rhodiola, have enough randomized trials to evaluate.

How they are thought to work

The best-supported idea is that these herbs act on the HPA axis - the hypothalamus-pituitary-adrenal loop that releases cortisol in response to stress. Ashwagandha’s withanolides appear to dampen the output of that loop, which fits the lower morning cortisol seen in trials. Rhodiola’s rosavins and salidroside are thought to work further upstream, changing how quickly the stress response switches on and off; in laboratory assays they also weakly inhibit the enzymes that break down serotonin and dopamine. Both are plausible mechanisms drawn from cell and animal work. Human trials measure cortisol and questionnaire scores, not the mechanism itself.

Ashwagandha: the best-evidenced of the group

Withania somnifera (ashwagandha) has the most supportive human data. A systematic review and meta-analysis (15 studies, 873 participants) found significant reductions in stress and cortisol after 8 weeks (meta-analysis), and multiple randomized, placebo-controlled trials report lower anxiety scores and morning cortisol with good tolerability (RCT). Most trials ran about 8 weeks using standardized root-extract doses.

The size of the effect in that meta-analysis: at 8 weeks, ashwagandha lowered Hamilton anxiety scores by about 3.5 points and Perceived Stress Scale scores by about 4.9 points more than placebo, alongside a significant fall in cortisol; quality-of-life scores did not improve significantly. An earlier meta-analysis of 12 trials (1,002 participants) found large pooled effects on anxiety and stress but rated the certainty of the evidence low, with the stress benefit concentrated at 300–600 mg per day (Akhgarjand 2022).

The individual trials behind those numbers are small and mostly from India:

• 64 adults with chronic stress, 300 mg of a high-concentration root extract twice daily for 60 days: stress scores fell significantly versus placebo and serum cortisol dropped substantially (P = 0.0006); adverse effects were mild and similar to placebo (Chandrasekhar 2012).
• 60 stressed adults, 240 mg of a standardized extract once daily for 60 days: Hamilton anxiety scores fell (P = 0.040) and morning cortisol fell (P < 0.001) versus placebo, though the broader DASS-21 stress score only approached significance (Lopresti 2019).
• 60 stressed adults, 250 or 600 mg per day for 8 weeks: perceived stress and serum cortisol fell at both doses, most clearly at 600 mg, and sleep quality improved versus placebo (Salve 2019).

Where the ashwagandha evidence is weak

A 2025 meta-analysis that restricted itself to trials reporting both cortisol and the Perceived Stress Scale (seven and six studies respectively, 488 participants) found cortisol fell by about 1.2 µg/dL but perceived stress did not change significantly (Albalawi 2025). Different inclusion rules, different answer - which tells you the evidence base is small enough that a handful of trials swing the result. Trials are short (about 8 weeks), samples are 50–64 people, some have ties to extract manufacturers, and there is no long-term safety data. The ashwagandha deep dive covers the sleep, testosterone and thyroid trials in more detail.

Rhodiola and the rest

Rhodiola rosea is popular for fatigue and mental stamina; the evidence is promising but thinner and more mixed than ashwagandha’s (a 2012 systematic review found most trials small and at risk of bias — Ishaque et al., BMC Complement Altern Med). That review found 11 trials; two of six on physical fatigue and three of five on mental fatigue were positive, and every one had a high or unclear risk of bias (Ishaque 2012).

The best single trial is Olsson’s: 60 adults meeting Swedish criteria for stress-related fatigue took 576 mg per day of the SHR-5 extract or placebo for 28 days. Burnout scores and attention improved more on rhodiola, and the cortisol awakening response fell (Olsson 2009). The most rigorous test, though, was a 12-week trial comparing rhodiola with sertraline and placebo in 57 adults with mild-to-moderate depression: rhodiola was not significantly better than placebo and less effective than sertraline, though with fewer side effects (30% versus 63%) (Mao 2015). Rhodiola, holy basil, and others may help some people, but quality trials are fewer. The rhodiola guide goes deeper on the fatigue trials.

What the studies used

These are the conditions in the trials, not a recommendation; talk to a clinician before starting, especially if you take medication, are pregnant, or have a diagnosed condition.

• Ashwagandha: standardized root extracts at 240–600 mg per day, in one or two daily doses, for 8 weeks or 60 days; the dose-comparison trial measured stress scores at 4 and 8 weeks.
• Rhodiola: standardized root extracts; the fatigue trial used 576 mg per day for four weeks and the depression trial ran for twelve weeks.
• What to expect: the trial effects are modest shifts on stress questionnaires, not a transformation. If nothing has changed after 8 weeks, the trials give no reason to continue.

Safety - read this part

• Ashwagandha is in the nightshade family, and there have been case reports of liver injury; avoid it if you have liver disease. A case series from Iceland and the US Drug-Induced Liver Injury Network described five people who developed jaundice 2–12 weeks after starting ashwagandha supplements; the injury was cholestatic or mixed, liver tests normalised within 1–5 months in the four patients followed up, and chemical analysis confirmed ashwagandha with no other toxic compounds (in one case rhodiola was a possible contributor) (Björnsson 2020). In an Indian series of eight single-ingredient cases, five patients had pre-existing chronic liver disease and three of them died of acute-on-chronic liver failure (Philips 2023).
• It can affect thyroid hormone levels and may interact with thyroid, sedative, immunosuppressant, and diabetes medications. In a placebo-controlled trial of 50 people with subclinical hypothyroidism, 600 mg per day for 8 weeks significantly shifted TSH, T3 and T4 (Sharma 2018) - a reason for anyone on thyroid medication, or with an overactive thyroid, to avoid it unless a clinician is monitoring.
• Avoid in pregnancy (traditionally considered an abortifacient) and use caution with autoimmune or hormone-sensitive conditions.
• Rhodiola was well tolerated in the trials above (fewer side effects than sertraline in the depression trial), but its weak MAO-inhibiting activity in laboratory assays is a reason for caution alongside antidepressants.
• As with all supplements, the FDA does not verify potency or purity before sale - choose third-party-tested brands.

Who should talk to a clinician first. Anyone with liver disease or abnormal liver tests; anyone on thyroid, sedative, immunosuppressant, diabetes or antidepressant medication; anyone pregnant, breastfeeding or trying to conceive; and anyone whose “stress” is persistent low mood, panic or sleeplessness that is affecting daily life - those need assessment, not an herb. Stop and seek care for yellowing skin or eyes, dark urine, itching or unusual fatigue.

Bottom line

Ashwagandha has real, if modest, short-term evidence for stress and cortisol; rhodiola is a maybe. They are tools, not magic, and they carry genuine interactions and contraindications, so talk to a clinician before starting - especially if you take medication or are pregnant.