Ashwagandha: Stress, Cortisol, and Adaptogen Science

Ashwagandha (Withania somnifera) is the most-studied plant in the Ayurvedic tradition for stress and anxiety, and the only adaptogen with multiple randomized controlled trials in modern medical literature. The two trials most often cited are Chandrasekhar and colleagues' 2012 study on chronic stress and cortisol, published in the Indian Journal of Psychological Medicine, and Lopresti and colleagues' 2019 study on stress, sleep, and quality of life, published in Medicine. Both found meaningful reductions in stress markers and self-reported anxiety. The picture is genuinely promising, but the supplement industry around ashwagandha is messy, with serious quality and standardization issues. This guide covers what the trials showed, how to choose a product, dosing, and the safety profile.

What ashwagandha is and what an adaptogen means

Ashwagandha is a small evergreen shrub in the nightshade family, native to India, North Africa, and the Middle East. The root is the part used medicinally. It contains a group of steroidal lactones called withanolides (notably withaferin A and withanolide D) that are believed to be the primary active compounds, along with alkaloids and saponins. Withanolides have documented effects on the hypothalamic-pituitary-adrenal (HPA) axis, GABAergic signaling, and inflammatory pathways in animal and cellular studies.

"Adaptogen" is a functional category, not a chemical one. The term was coined by Soviet scientists in the 1940s to describe substances that increase resistance to stress and have a normalizing effect on physiology, regardless of the direction of the disturbance. The category includes ashwagandha, rhodiola, panax ginseng, holy basil, and eleuthero. The concept is loose enough that it can be hard to pin down mechanistically, but the common thread is modulation of the stress response, particularly the HPA axis and cortisol. Ashwagandha has the strongest human trial data of any adaptogen for stress and anxiety.

The Chandrasekhar 2012 cortisol trial

The most influential ashwagandha trial for stress and cortisol was published by Chandrasekhar and colleagues in 2012 in the Indian Journal of Psychological Medicine. They recruited 64 adults with chronic stress (defined as a Hamilton Anxiety Scale score above 14) and randomized them to 300 mg twice per day of a standardized ashwagandha root extract (KSM-66, standardized to 5 percent withanolides) or placebo for 60 days. The primary outcomes were the Perceived Stress Scale (PSS) and serum cortisol.

The results were striking:

• PSS scores dropped by 44 percent in the ashwagandha group versus 5.5 percent in placebo, a highly significant difference.
• Serum cortisol dropped by 27.9 percent in the ashwagandha group versus 7.9 percent in placebo.
• All five stress-assessment subscales (pessimism, fatigue, anxiety, distraction, irritability) improved significantly more in the treatment group.
• No serious adverse events were reported, and the dropout rate was similar between groups.

The 28 percent cortisol reduction is notable because cortisol is a regulated hormone, and most interventions that affect it (meditation, exercise, sleep) produce 10 to 20 percent reductions at the high end. A 28 percent reduction from a single supplement is unusual. The mechanism is thought to be HPA-axis modulation: ashwagandha appears to dampen the cortisol response to stressors without suppressing basal cortisol, leaving the diurnal rhythm intact.

The Lopresti 2019 stress and sleep trial

Lopresti and colleagues published a more comprehensive trial in 2019 in the journal Medicine. They randomized 138 adults with self-reported stress and anxiety to one of three arms: 250 mg per day of ashwagandha extract (KSM-66), 600 mg per day, or placebo, for 8 weeks. The primary outcomes were stress (DASS-21), anxiety, sleep quality (the Pittsburgh Sleep Quality Index), and quality of life.

The findings:

• Stress (DASS-21) dropped more in both ashwagandha groups than placebo, with a dose-response: the 600 mg group showed a larger reduction than the 250 mg group.
• Anxiety scores dropped more in the ashwagandha groups.
• Sleep quality improved in both ashwagandha groups, with the 600 mg group showing the largest improvement in PSQI scores.
• Quality of life measures improved in the treatment groups.
• Serum cortisol and DHEA-S were measured in a subset, with cortisol reductions consistent with the Chandrasekhar trial.

The Lopresti trial is important because it confirms the Chandrasekhar findings in a larger sample, with a dose-response relationship, and extends the benefits to sleep quality. The 600 mg per day dose of KSM-66 appears to be the effective threshold for most outcomes. The trial also reports the effect sizes in standard deviation units (Cohen's d), which were in the moderate range (0.4 to 0.7) for most outcomes, meaningful but not transformative.

Other trials and the broader picture

A 2022 meta-analysis in Journal of Herbal Medicine pooled 12 randomized trials of ashwagandha for stress and anxiety, totaling about 1,000 participants. The pooled analysis found that ashwagandha produced significant reductions in self-reported stress and anxiety compared to placebo, with effect sizes in the small to moderate range (Cohen's d around 0.4 to 0.5). Cortisol reductions were consistent across trials, averaging about 20 to 25 percent. The authors noted that the trials were generally of moderate quality, with risk of bias from small samples and industry funding in several cases.

A small but interesting set of trials has examined ashwagandha for athletic performance. A 2015 trial in Journal of the International Society of Sports Nutrition gave 300 mg twice per day to elite cyclists for 8 weeks and found improvements in VO2 max and time to exhaustion. A 2018 trial in the same journal gave 600 mg per day to recreationally active men for 12 weeks and found improvements in strength and testosterone. The performance data is suggestive but smaller in volume than the stress data.

The withanolide question and product standardization

The single biggest problem in the ashwagandha market is product standardization. The withanolide content of raw ashwagandha root varies from 0.1 to 0.5 percent by weight, depending on the variety, growing conditions, and harvest time. To make a consistent product, manufacturers extract and standardize to a specific withanolide content. The two main standardized extracts used in trials are:

• KSM-66: A full-spectrum root extract standardized to 5 percent withanolides. Used in the Chandrasekhar 2012 and Lopresti 2019 trials. The most-studied form.
• Sensoril (also called NooGandh and others): A root-and-leaf extract standardized to 32 percent withanolides, with a different withanolide profile (more withanolide A, less withaferin A). Used in some trials and often preferred for stress and sleep.
• Shoden: A newer extract standardized to 35 percent withanolides, with limited trial data.

The withanolide percentage is a useful rough guide but it does not capture which withanolides are present. Withaferin A is more cytotoxic and may be responsible for some of the negative side effects at high doses. Withanolide A is more associated with neuroprotection. A 5 percent KSM-66 extract and a 35 percent Shoden extract are not interchangeable. If you are following a specific trial protocol, use the extract that was used in that trial. The KSM-66 trials are the most reproducible because the product is widely available and the dosing is well-defined.

A 2021 study in Frontiers in Pharmacology tested 11 commercial ashwagandha supplements and found that 5 contained less than 50 percent of the labeled withanolide content, and 2 contained no detectable withanolides at all. Buy only from suppliers with third-party testing and certificates of analysis.

Dosing

The dosing used in the trials:

• For stress and cortisol: 300 mg twice per day of KSM-66 (600 mg total) for at least 8 weeks. Chandrasekhar used 300 mg twice per day for 60 days; Lopresti used 600 mg per day for 8 weeks. Both produced significant effects.
• For sleep: 300 to 600 mg per day of KSM-66, taken in the evening. The Lopresti trial found the 600 mg dose more effective for sleep.
• For athletic performance: 300 mg twice per day (600 mg total) for 8 to 12 weeks.

Take with food. The withanolides are lipophilic and absorption improves with a meal containing fat. Most trials do not specify timing relative to meals, but spreading the dose across the day (morning and evening) appears to work better than a single large dose.

Allow at least 4 weeks for the effect to manifest. The Chandrasekhar trial measured significant cortisol reduction by day 30, with continued improvement to day 60. The Lopresti trial saw similar time courses. If you feel nothing after 4 weeks, the dose is likely too low or the product is poorly standardized.

Safety and drug interactions

Ashwagandha is generally well tolerated at trial doses. The most common side effects are mild gastrointestinal upset, drowsiness, and headache. The more serious concerns are:

• Thyroid effects. Ashwagandha may increase thyroid hormone levels, particularly T4. This is potentially beneficial for people with subclinical hypothyroidism but risky for people with hyperthyroidism or on thyroid medication. A 2018 trial in Journal of Alternative and Complementary Medicine found that ashwagandha normalized thyroid markers in subclinical hypothyroid patients. Discuss with an endocrinologist if you have thyroid disease.
• Sedative interaction. Ashwagandha has GABAergic activity and can add to the effects of sedatives, benzodiazepines, and alcohol. Avoid combining with CNS depressants.
• Immune modulation. Ashwagandha may stimulate the immune system, which could be problematic for people with autoimmune disease or on immunosuppressive drugs.
• Pregnancy and breastfeeding. Avoid. Ashwagandha may have abortifacient effects based on animal studies.
• Blood sugar. Ashwagandha may lower blood glucose, which could be additive with diabetes medications.

Stop ashwagandha 2 weeks before surgery due to potential interactions with anesthesia and blood sugar. The Memorial Sloan Kettering About Herbs database maintains the most accessible summary of these interactions for clinicians.

Who should consider ashwagandha

The strongest trial support is for adults with chronic stress, elevated anxiety, or sleep disturbance related to stress. If you have a Hamilton Anxiety Scale score above 14 or a Perceived Stress Scale score above 20, you fit the trial population. Use the Stress Index Calculator to get a rough sense of your baseline.

Ashwagandha is not a substitute for the foundational interventions: sleep, exercise, social connection, and therapy or medication for clinical anxiety or depression. It is an adjunct that may produce a 20 to 30 percent reduction in stress markers and symptoms, on top of the basics. A 2023 narrative review in Phytotherapy Research noted that ashwagandha is "best viewed as a low-risk adjunct to lifestyle interventions for stress-related conditions, not as a stand-alone treatment."

For athletes, the evidence is suggestive but smaller. The 2015 and 2018 trials showed performance benefits, but the literature is not as robust as for stress. Try it if you are otherwise optimizing training and want a marginal gain.

Common pitfalls

Pitfall 1: Buying cheap, unstandardized ashwagandha

The withanolide content of raw ashwagandha is too variable to dose reliably. A 2021 study found that nearly half of commercial products were underdosed. Buy only standardized extracts (KSM-66, Sensoril, or Shoden) from suppliers with certificates of analysis. The cheap bulk powder on Amazon is not the same as the standardized extract used in trials.

Pitfall 2: Expecting immediate effects

Ashwagandha is not a benzodiazepine. The cortisol and stress effects take 4 to 8 weeks to manifest. People who try it for a week and conclude it does nothing are not following the trial protocol. Give it at least 8 weeks at the right dose before evaluating.

Pitfall 3: Taking it on an empty stomach

Withanolides are lipophilic. Taking ashwagandha on an empty stomach may reduce absorption and increase gastric side effects. Take with a meal containing some fat.

Pitfall 4: Using it as a substitute for therapy or medication

For clinical anxiety or depression, ashwagandha is not a first-line treatment. The effect sizes are moderate, not large. If you have a clinical diagnosis, discuss with your psychiatrist or therapist before adding ashwagandha, especially if you are on SSRIs, SNRIs, or benzodiazepines, because of potential serotonin and GABA interactions.

Pitfall 5: Staying on it indefinitely

The trials ran for 8 to 12 weeks. Long-term safety data on continuous use for years is limited. A reasonable approach is to use ashwagandha for 8 to 12 weeks during a period of acute stress, then taper off and see how you do without it. Cycle off for at least a month every few months. Track your stress and sleep with the food diary during both on and off periods to see if the effect persists.

The bottom line

Ashwagandha is the best-studied adaptogen for stress, anxiety, and sleep, with multiple randomized trials showing 20 to 30 percent reductions in cortisol and stress symptoms. The Chandrasekhar 2012 trial (300 mg twice per day of KSM-66 for 60 days) and the Lopresti 2019 trial (600 mg per day for 8 weeks) are the key references. The effective dose is 300 to 600 mg per day of standardized KSM-66 extract, taken with food, for at least 8 weeks. Product quality is the dominant variable: buy only standardized extracts with third-party testing. Ashwagandha is an adjunct to the foundational interventions of sleep, exercise, social connection, and clinical care, not a replacement. Use the Stress Index Calculator and Sleep Debt Calculator to quantify the effect, and discuss with your doctor if you have thyroid disease, autoimmune disease, or are on sedatives or thyroid medication.