Ashwagandha (Withania somnifera) is an herb used in Ayurveda, the traditional medicine of India. Its root has a horsey smell (in Sanskrit, ashva means “horse” and gandha means “smell”) and is said to confer the strength and virility of a horse. Various parts of the plant are used, but the most common in supplements is an extract of its roots.
A number of studies suggest that it has anxiolytic (anti-anxiety) effects; studies are mostly supportive of a notable effect of ashwagandha for this purpose, and it seems to reduce cortisol levels. However, more research is needed before we can have a great deal of confidence in it or know the optimal dose. Ashwagandha may also be able to reduce insomnia, fatigue, and the symptoms of depression, but it hasn't been well-researched for these purposes.
It may increase power output in untrained subjects during resistance exercise and anaerobic running, but this observation is based on a small amount of research and more is needed. It may lead to small reductions in blood glucose, blood pressure, and LDL-cholesterol, while slightly increasing HDL-cholesterol. Additionally, it may modestly increase testosterone and sperm quality in infertile men.
It seems to be safe, but it needs more long-term research that's specifically designed to evaluate its safety. It may cause mild drowsiness and sedation for some people.
This question is difficult to answer because we currently lack a lot of long-term information from clinical trials. Because of its possible drug-like effects on neurotransmission, it's difficult to rule out unintended side-effects or a loss of potency in the long-term. However, what studies we do have generally suggest that the effects of Ashwagandha on stress/anxiety continually improve for at least for two months after the beginning of daily supplementation. Furthermore, the studies use Ashwagandha daily, and if a study finds an effect, the best way to get that same effect is to mimic the dose and dosing schedule in the study. It is unknown if taking breaks from Ashwagandha or taking it every other day will yield the same effects.
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