Ashwagandha has quietly become one of the best-selling herbal supplements in the United States, showing up in gummies, protein powders, and “calm” stacks aimed at everyone from stressed-out office workers to competitive lifters. A new evidence review from Harvard Health takes a more measured look, and it lands somewhere most marketing copy never goes: real benefits, backed by real trials, alongside a growing list of case reports doctors are watching closely.
What the Research Actually Supports
Ashwagandha (Withania somnifera) is an adaptogenic herb used for centuries in Ayurvedic medicine, and modern trials have started to catch up with the tradition. The strongest evidence sits around stress and anxiety, where standardized root extracts show measurable effects in randomized, placebo-controlled trials rather than anecdote.
In one widely cited trial published in the Indian Journal of Psychological Medicine, adults who took 300 mg of a high-concentration ashwagandha extract twice daily for 60 days saw serum cortisol drop by nearly 28 percent, compared with an 8 percent drop in the placebo group. Self-reported stress scores fell in step, and the extract was generally well tolerated at that dose.
A second placebo-controlled trial, published in the journal Medicine, found similar reductions in perceived stress and improvements in sleep quality after eight weeks of daily use. Neither study found ashwagandha to be sedating or impairing, which is part of why it gets marketed as a daytime option rather than a sleep aid.
The Strength and Recovery Claims Hold Up, With Caveats
Ashwagandha’s reputation in fitness circles rests largely on one well-designed trial in the Journal of the International Society of Sports Nutrition. Men who combined resistance training with 600 mg of ashwagandha root extract daily for eight weeks gained more muscle strength and had greater reductions in exercise-induced muscle damage than the training-only placebo group, along with a modest rise in testosterone.
That is a real, measurable effect, but it comes from a single trial in young, healthy men doing structured resistance training. Harvard Health’s review is careful to note that the supplement is not a substitute for progressive overload, adequate protein, or sleep, all of which did more heavy lifting in that same study than the herb itself.
The Liver Question Doctors Are Now Watching
The part of the review getting the most attention is the safety section, specifically a cluster of case reports describing liver injury in people taking ashwagandha supplements. Iceland’s national drug agency and the U.S. Drug-Induced Liver Injury Network have both documented cases in which patients developed jaundice, elevated liver enzymes, or acute hepatitis after starting ashwagandha, with symptoms resolving after they stopped taking it.
These remain rare events relative to how many people take the supplement without issue, and case reports cannot prove causation the way a controlled trial can. But regulators flagged the pattern for a reason: unlike a randomized trial, a case series exists precisely because clinicians kept seeing the same unusual thing happen to different patients on the same substance.
Harvard Health’s guidance is straightforward: anyone with existing liver disease, anyone on medications processed by the liver, and anyone who notices unusual fatigue, dark urine, or yellowing skin after starting ashwagandha should stop and talk to a doctor rather than pushing through it.
Thyroid Effects Cut Both Ways
Ashwagandha has a documented effect on thyroid hormone levels, which is a feature for some people and a risk for others. A randomized trial in the Journal of Alternative and Complementary Medicine found that ashwagandha extract meaningfully improved thyroid-stimulating hormone and T4 levels in adults with subclinical hypothyroidism over eight weeks.
That same mechanism is exactly why Harvard Health’s review flags caution for people with hyperthyroidism or Hashimoto’s disease already on thyroid medication. Nudging thyroid hormone levels in someone who is already overactive, or already medicated to a stable level, can push them out of range rather than into it.
Who Should Skip It Entirely
Pregnant women top the list. Animal studies have linked ashwagandha to increased risk of miscarriage, and there is no human safety data solid enough to override that caution, so most obstetric guidance says to avoid it during pregnancy.
People with autoimmune conditions such as rheumatoid arthritis, lupus, or type 1 diabetes should also talk to a doctor first, since ashwagandha’s immune-stimulating properties could theoretically increase disease activity. The same caution applies to anyone scheduled for surgery, because the herb can affect blood sugar and central nervous system activity in ways that complicate anesthesia, and most surgeons recommend stopping it at least two weeks beforehand.
How Much Is Actually Being Studied
Most of the positive trial data clusters around 300 to 600 mg per day of a standardized root extract, taken for eight to twelve weeks, not the higher doses or indefinite daily use some products encourage. Harvard Health’s review points out that “more is better” has never been the finding in these trials, and higher doses simply have not been tested with the same rigor.
Extract standardization matters too. The trials cited above used specific, tested formulations (KSM-66 and Sensoril among them), and a bottle that lists only “ashwagandha root powder” without a standardized extract percentage is not the same product that produced these results.
The Bottom Line
Ashwagandha is one of the few adaptogens with genuine randomized trial support behind its stress and strength claims, which puts it ahead of most of the supplement aisle. But it is not risk-free, and the same properties that make it effective, its influence on cortisol, thyroid hormone, and immune activity, are the reasons it needs real screening before use, not just a quick label check.
The people best served by ashwagandha are healthy adults without liver, thyroid, or autoimmune complications, using a standardized extract at a studied dose for a defined stretch of time. Everyone else has homework to do first, and Harvard Health’s review is a useful reminder that “natural” was never the same claim as “harmless.”
