Verdict: Promising but thin evidence; training will do far more, and safety questions are real
Safety: Rare but serious liver injury reported; may affect thyroid hormones; avoid in pregnancy
Key takeaways
A 2020 meta-analysis of four small trials found ashwagandha raised VO2max by about 3 ml/kg/min, with high heterogeneity
Most trials are short, small and use branded extracts, often with manufacturer involvement
Rare cases of liver injury, thyroid effects and a Danish ban mean it is not a risk-free supplement
Ashwagandha (Withania somnifera) is an Ayurvedic root marketed for stress, sleep, testosterone and, more recently, endurance. Searches like "ashwagandha VO2max" and "is ashwagandha good for working out" have exploded. The evidence is more interesting than for most adaptogens, but it is still thin, and safety concerns have grown.
What it is
Ashwagandha is a shrub whose root (and sometimes leaf) is dried and extracted. Supplements are usually standardised to withanolides, the compounds thought to be active. Most sports trials use branded root extracts sold by the same companies that fund or supply the research, which is worth keeping in mind when reading the results.
Does it raise VO2max?
The key paper is a systematic review and meta-analysis by Pérez-Gómez and colleagues (Nutrients, 2020). They found five randomised trials (162 participants) and pooled four of them (142 participants). Ashwagandha raised VO2max by a mean of 3.00 ml/kg/min (95% CI 0.18 to 5.82) compared with placebo, in both athletes and non-athletes.
That sounds meaningful — similar in size to gains often reported after a few weeks of interval training in a beginner. But the authors themselves concluded that ashwagandha "might" improve VO2max, because the number of studies was small and heterogeneity was high. The lower end of the confidence interval is close to zero. A later Bayesian meta-analysis by Bonilla and colleagues (2021) pooled 12 trials across strength, cardio and recovery outcomes and also found ashwagandha outperformed placebo, while rating risk of bias low to moderate.
Points to weigh:
- Few, small trials. Four studies is not much for a pooled estimate.
- Short durations. Usually 8–12 weeks, often in untrained or lightly trained people who would improve anyway.
- Industry links. Many trials test a single branded extract with support from the maker.
- No mechanism for oxygen delivery. VO2max is limited mainly by the heart and blood (see what limits VO2max). It is not clear how a root extract would change that, other than indirectly through better recovery or sleep.
Does it improve endurance performance?
Possibly a little, but no large trial shows faster race times in trained endurance athletes. Most outcomes are lab measures such as VO2max, time to exhaustion or strength. Any effect is far smaller than what structured training delivers: a block of Norwegian 4x4 intervals is a better-proven way to raise VO2max.
How it is used in studies
Trials typically use about 300–600 mg a day of a standardised root extract for 8–12 weeks, often split into two doses. Effects, where reported, appeared after weeks of daily use rather than from a single dose, so ashwagandha is not a pre-race supplement. Products vary widely in withanolide content, plant part (root vs leaf) and extraction method, so results from one extract do not automatically apply to another.
Side effects, safety and anti-doping
Ashwagandha is generally well tolerated in short trials, with mild stomach upset or drowsiness the usual complaints. The rarer risks are more serious:
- Liver injury. Case series from Iceland and the US Drug-Induced Liver Injury Network (Björnsson et al., Liver International 2020) and a review of reported cases (Bokan et al., 2023) describe cholestatic, mixed or hepatocellular liver injury, sometimes severe, typically weeks to months after starting. It is rare, but stop and seek care if you get jaundice, itching or dark urine.
- Thyroid. In a small trial in people with subclinical hypothyroidism (Sharma et al., 2018), ashwagandha significantly changed TSH, T3 and T4 levels. That may sound useful, but it means it can interfere with thyroid conditions and their treatment.
- Pregnancy and regulation. Denmark banned ashwagandha in supplements in 2023 after a risk assessment raised concerns about hormonal effects and possible abortifacient properties. Avoid it if pregnant or trying to conceive.
- Interactions. It may add to the effect of sedatives, thyroid medication and drugs that affect the immune system or blood sugar.
Ashwagandha is not on the WADA prohibited list, but herbal supplements carry a contamination risk. If you compete, choose a third-party tested product (for example, Informed Sport or NSF Certified for Sport).
This page is not medical advice. Talk to a doctor before using ashwagandha if you have a liver or thyroid condition, are pregnant, or take any medication.
Who might benefit
People curious about stress and sleep who are already training consistently, have no liver or thyroid issues and are not pregnant. Even then, expect small or no changes in fitness. If you try it, keep your training steady and measure the same way before and after, otherwise any change in a watch estimate is impossible to attribute. If your goal is a higher VO2max, start with how to improve VO2max and track progress with the VO2max calculator.
Bottom line
A small meta-analysis suggests ashwagandha might raise VO2max by around 3 ml/kg/min, but it rests on a handful of short, often industry-linked trials. Rare liver injury and hormonal effects mean it is not a harmless add-on. For endurance, caffeine and beetroot nitrate have much better evidence, and training beats all of them. See the full supplements guide.
Frequently asked questions
Does ashwagandha increase VO2max?
A 2020 meta-analysis of four small randomised trials found an average gain of about 3 ml/kg/min, but the authors called the evidence limited and the results inconsistent. Treat it as possible, not proven.
Is ashwagandha good for working out?
Some small trials report better strength, endurance and recovery, but most are short, use branded extracts and are often linked to the manufacturer. Consistent training, sleep and food matter much more.
What are the benefits of ashwagandha for runners?
The best case is a small rise in VO2max and less perceived stress, but no large trial in trained runners shows faster race times.
Is ashwagandha safe?
Most people in trials tolerated it, but rare cases of liver injury have been reported, it can change thyroid hormone levels, and it should be avoided in pregnancy. Talk to a doctor first if you have a condition or take medication.
Sources
- Pérez-Gómez J, Villafaina S, Adsuar JC, Merellano-Navarro E, Collado-Mateo D. Effects of Ashwagandha (Withania somnifera) on VO2max: a systematic review and meta-analysis. Nutrients. 2020;12(4):1119.
- Bonilla DA, Moreno Y, Gho C, Petro JL, Odriozola-Martínez A, Kreider RB. Effects of Ashwagandha (Withania somnifera) on physical performance: systematic review and Bayesian meta-analysis. J Funct Morphol Kinesiol. 2021;6(1):20.
- Björnsson HK, Björnsson ES, Avula B, et al. Ashwagandha-induced liver injury: a case series from Iceland and the US Drug-Induced Liver Injury Network. Liver Int. 2020;40(4):825–829.
- Bokan G, Glamočanin T, Mavija Z, et al. Herb-induced liver injury by Ayurvedic ashwagandha as assessed for causality by the updated RUCAM: an emerging cause. Pharmaceuticals (Basel). 2023;16(8):1129.
- Sharma AK, Basu I, Singh S. Efficacy and safety of ashwagandha root extract in subclinical hypothyroid patients: a double-blind, randomized placebo-controlled trial. J Altern Complement Med. 2018;24(3):243–248.
- Danish ban on Ashwagandha: truth, evidence, ethics, and regulations. AYU, 2024. Article (critique of the Danish risk assessment)
- Vitafoods Insights. Industry responds to the Danish ashwagandha assessment, 2023. Article