Ashwagandha is a common name for the plant Withania somnifera, which is also known as "winter cherry" and "Indian ginseng." It is used in Indian traditional medicine to treat anxiety, chronic inflammation, depression, fatigue, stress, and weakness. While it is also used to improve cognition, the evidence in humans is very limited and it is unknown if ashwagandha can slow cognitive decline or prevent Alzheimer's disease. Ashwagandha is likely safe for most people though it has some drug interactions and should not be used during pregnancy.
A few studies have examined short-term cognitive effects of ashwagandha, but no studies have tested whether it can prevent age-related cognitive decline or dementia. Our search identified:
• 0 clinical studies in humans examining the treatment or prevention of cognitive decline or dementia
• 1 randomized controlled clinical trial in healthy adults examining the effects on cognitive function
• 1 randomized controlled clinical trial in bipolar disorder patients
• Numerous preclinical studies on possible mechanisms of action
It is currently unknown whether ashwagandha can prevent cognitive decline or dementia in humans. In a small double-blind placebo-controlled clinical trial, supplementation at 250 mg twice daily for 2 weeks improved psychomotor performance (reaction time) in healthy men . In another randomized controlled study, patients with bipolar disorder treated with 500 mg/day experienced greater improvements in a type of working memory, a measure of reaction time, and a measure of social cognition, compared to those receiving placebo . However, no benefits with ashwagandha were seen in other cognitive measures including executive function, processing speed, and psychomotor speed. Both of these trials were too small and short to determine whether ashwagandha has long-term benefits for cognitive health. Laboratory studies have shown that ashwagandha may benefit cognitive function by promoting neuronal growth and protecting neurons from damage and oxidative stress [4-7], but these benefits have not been confirmed in humans.
It is currently unknown whether ashwagandha can improve cognition or slow cognitive decline in people with dementia. While some benefits have been observed in preclinical studies of Alzheimer's disease , ashwagandha has not been clinically studied in people with dementia.
Ashwagandha has a long history of use in traditional Indian Ayurvedic medicine, and it is generally considered safe when used properly in healthy people. However, ashwagandha can decrease blood sugar levels, lower blood pressure, and increase thyroid hormone levels, and therefore people who have those conditions or people taking medications for diabetes, high blood pressure, or thyroid conditions should exercise caution . Ashwagandha should not be used during pregnancy as it may cause miscarriages. More information on safety and drug interactions with ashwagandha can be found on MedlinePlus.
Long-term safety has not been studied in clinical research but small clinical trials lasting 30–60 days with daily doses ranging from 300–1250 mg have reported that ashwagandha is well-tolerated with few side effects .
NOTE: This is not a comprehensive safety evaluation or complete list of potentially harmful drug interactions. It is important to discuss safety issues with your physician before taking any new supplement or medication.
Ashwagandha is taken orally, either in pill form or powdered and mixed with liquids such as tea. Typical doses range from 250–2000 mg/day. Supplements commonly contain the root of the plant described either as "whole" or "extract." The extracts obtained using water versus alcohol can contain different chemicals . However, which specific chemicals are bioactive is still controversial  and the available scientific evidence is inadequate to predict whether the effects would be better in whole versus extracted ashwagandha root or water versus alcohol extracts. Some supplements sell preparations of the plant's leaves but, according to the Ayurvedic pharmacopoeia of India, ashwagandha should be prepared from the roots . As with most supplements, the content can vary depending on the bottle and brand, for example with the risk of toxins added during processing or contaminants such as heavy metals  if the plant was grown in a polluted area.
Additional information and other purported uses of ashwagandha can be found at the Memorial Sloan Kettering Cancer Center.
Quality Control of Sources: United States Pharmacopeial Convention (USP) and ConsumerLab offer information on the quality of specific supplements and can assist in finding a trusted brand.
More information on safety and drug interactions with ashwagandha at MedlinePlus