Ashwagandha has the strongest research base of the commonly marketed testosterone-support ingredients, but strongest doesn’t mean risk-free. A fair look at this ingredient means holding two things that are both true: large clinical trials have generally found it well tolerated, and a small number of concerning case reports involving liver and thyroid effects genuinely exist and deserve to be taken seriously rather than dismissed.
What Controlled Clinical Trials Actually Found
A 2026 systematic review pooled data from 23 randomized controlled trials involving more than 2,300 healthy adults who took standardized ashwagandha root extract, at doses ranging from 125 to 600 mg per day, for periods up to 180 days. Across these trials, key liver enzymes, kidney function markers, and thyroid measures remained within established normal ranges with a consistent pattern, and the researchers rated the evidence for these core safety biomarkers as moderate certainty, reflecting fairly consistent results across multiple well-designed trials (Systematic review, 2026, Nutrients). This is a meaningfully reassuring body of evidence, and it reflects what most people taking a standardized, single-ingredient ashwagandha product are likely to experience.
When a supplement label lists “ashwagandha” without any further detail, it’s worth knowing that the research behind this herb is overwhelmingly concentrated on one specific, branded extract rather than the plant in general. KSM-66 is that extract, and it’s been used in more published clinical trials than any other ashwagandha formulation on the market, including a study that specifically measured its effect on testosterone in men engaged in resistance training and found a significantly greater increase compared to placebo (Wankhede et al., 2015, Journal of the International Society of Sports Nutrition). A generic ashwagandha extract without this specific standardization hasn’t necessarily been tested under the same conditions, which makes it difficult to know whether a given product would produce a similar result. Choosing a product that names KSM-66 specifically, rather than just “ashwagandha,” is a reasonable way to increase the odds that what you’re taking resembles what was actually studied. The testosterone-boosting supplement Testosil features KSM-66 ashwagandha as one of its 12 ingredients.
Milder, More Common Side Effects
The same body of trial evidence does report some milder side effects showing up more often than nothing at all. A separate 2024 systematic review found that while most randomized controlled trials reported no adverse events, several noted issues like diarrhea, nausea, abdominal discomfort, mild rashes, or occasional abnormal liver or kidney test results (cited in a review summarizing ashwagandha safety literature). Drowsiness or mild sedation has also been reported in some trials, particularly at higher doses.
What the Case Reports Show, and Why They’re Different From Trial Data
Separately from the controlled trial evidence, a smaller number of individual case reports have described more serious liver-related problems in people taking ashwagandha, including hepatitis, abnormal liver function, and cholestasis, a condition involving impaired bile flow. The timing of these reported issues varied considerably, from as early as 3 weeks to as late as 10 months after starting use in different reports (Critical review, 2026, Phytotherapy Research). In the cases where the outcome was tracked, liver function generally returned to normal after stopping the product, though it’s worth being direct that a small number of more severe cases have been documented.
An Important Distinction: Single-Ingredient vs. Multi-Ingredient Products
One detail is worth taking seriously when evaluating this risk. Researchers reviewing these case reports have specifically noted that many involved non-standardized extracts or multi-ingredient formulations, products combining ashwagandha with several other compounds, rather than the single-ingredient, standardized extracts used in the controlled clinical trials that showed a stronger safety record. This doesn’t eliminate the concern, but it does suggest that product quality and formulation may meaningfully affect risk, and it’s a real argument for choosing a standardized, single-ingredient product with a named, tested extract over a blend with unclear individual ingredient amounts.
What About Thyroid Effects?
Ashwagandha’s effect on thyroid hormones has a genuinely mixed research picture. In people with underactive thyroid function to begin with, ashwagandha has generally been associated with improving thyroid markers back toward normal, similar to the deficiency-correction pattern seen with zinc and testosterone. In people with normal thyroid function, most research hasn’t found a meaningful effect. Cases of the opposite problem, ashwagandha triggering overactive thyroid symptoms, have been reported but appear to be rare, with one review identifying only a handful of such cases in the available literature (Clinical review, Nutrients).
What This Means in Practical Terms
The overall picture from controlled trials supports ashwagandha as generally well tolerated for most healthy adults at standard doses, and the systematic review data is reassuring on this point. At the same time, the case reports on liver injury are real and shouldn’t be waved away, even though they appear to represent a small number of instances relative to how widely this supplement is used.
A few practical takeaways follow directly from this evidence: choosing a standardized, single-ingredient extract with a known withanolide content rather than an unlabeled blend is a reasonable way to reduce risk, since that’s what the safety-supporting trial data is actually based on. If you have a pre-existing liver condition, thyroid disorder, or take medications processed by the liver, it’s worth discussing ashwagandha with a doctor before starting rather than assuming it’s automatically fine. And if you notice symptoms like unusual fatigue, yellowing of the skin or eyes, dark urine, or abdominal pain after starting any new supplement, stopping and getting it checked out is the right call regardless of how rare that outcome is expected to be.
Frequently Asked Questions
A few common questions on this topic, based on what current research actually supports.
Is Ashwagandha Safe to Take Long-Term?
Controlled trials up to 180 days have generally shown normal liver, kidney, and thyroid markers with standardized extracts, though long-term data beyond this window is more limited.
Can Ashwagandha Cause Liver Damage?
Case reports have documented liver injury in a small number of individuals, often resolving after stopping use. Many of these cases involved non-standardized or multi-ingredient products rather than single-ingredient standardized extracts.
Does Ashwagandha Affect Thyroid Function?
It tends to improve thyroid markers in people with underactive thyroid function, has little effect in people with normal thyroid function, and has only rarely been linked to overactive thyroid symptoms.
How Can I Reduce My Risk When Taking Ashwagandha?
Choosing a standardized, single-ingredient extract with a known withanolide content, avoiding unlabeled blends, and consulting a doctor if you have liver or thyroid conditions are reasonable ways to reduce risk based on the available evidence.
