The Short Answer
For most healthy adults at standard doses (300-600 mg/day of a standardized extract like KSM-66 or Sensoril), ashwagandha appears safe for short-to-medium-term use (up to 12 weeks, the longest duration studied in most clinical trials). But "appears safe in clinical trials" is not the same as "safe for everyone in all circumstances." There are real concerns about liver injury in some individuals, meaningful drug interactions, thyroid hormone effects, and specific populations who should not take it. The safety profile is better than many herbal supplements, but it is not clean enough to take without understanding the caveats.
What Clinical Trials Show About Side Effects
The most cited systematic review on ashwagandha safety is a 2021 paper in the Journal of Ethnopharmacology that analyzed adverse events across 41 clinical studies encompassing over 3,000 participants. The overall finding: ashwagandha was well-tolerated in most trials, with adverse event rates comparable to placebo groups. The most commonly reported side effects were mild:
- Gastrointestinal discomfort (nausea, diarrhea, stomach upset) - reported in 5-10% of participants
- Drowsiness or sedation - reported in some trials, particularly at higher doses
- Headache - occasional reports, not consistently above placebo rates
These are the trial-level findings, and they are reassuring as far as they go. The limitation is that most trials lasted 8-12 weeks with sample sizes of 50-100 people. This is sufficient to detect common short-term side effects but not sufficient to detect rare serious events or long-term effects. No one has published a large-scale safety study of ashwagandha use beyond 12 weeks.
The Liver Injury Concern
This is the most serious safety signal and deserves a direct discussion. The National Institutes of Health LiverTox database has documented multiple case reports of clinically apparent liver injury associated with ashwagandha use. A 2023 review in the American Journal of Gastroenterology identified five cases of ashwagandha-associated hepatotoxicity, with onset typically 2-12 weeks after starting supplementation. Liver enzyme elevations were significant (ALT 10-40x the upper limit of normal), and in at least one case, liver biopsy showed autoimmune-like hepatitis features.
Context matters here. Case reports do not prove causation - people who take supplements also eat food, take medications, and expose their livers to other insults. And the number of reported cases is small relative to the millions of people taking ashwagandha. But the pattern is consistent enough that the NIH includes ashwagandha in its database of supplements with documented hepatotoxicity risk.
Practical takeaway: if you have pre-existing liver disease, elevated liver enzymes, or are taking medications metabolized by the liver, ashwagandha is not worth the risk. If you are healthy and decide to take it, periodic liver function testing (a standard blood panel) is a reasonable precaution for long-term use.
Thyroid Effects: Real and Clinically Relevant
Ashwagandha has been shown to increase thyroid hormone levels. A 2018 randomized controlled trial published in the Journal of Alternative and Complementary Medicine found that 600 mg/day of ashwagandha root extract significantly increased serum T3 and T4 levels in subclinically hypothyroid adults over eight weeks. TSH levels decreased correspondingly.
For people with subclinical hypothyroidism, this could be beneficial (and is one reason some practitioners recommend it for thyroid support). But for people with hyperthyroidism, Graves' disease, or who are already on thyroid medication (levothyroxine), ashwagandha's thyroid-stimulating effect is a genuine risk. It can push thyroid hormones outside the therapeutic range and interfere with medication dosing.
If you have any thyroid condition or are taking thyroid medication, do not take ashwagandha without discussing it with your endocrinologist. This is not a theoretical concern - it is a documented pharmacological effect.
Drug Interactions
Ashwagandha has several clinically meaningful drug interactions:
- Thyroid medications (levothyroxine): As noted above, ashwagandha can alter thyroid hormone levels, potentially requiring dose adjustments
- Immunosuppressants: Ashwagandha has immune-modulating properties. It may counteract the effects of immunosuppressive drugs used after organ transplant or for autoimmune conditions
- Sedatives and benzodiazepines: Ashwagandha has GABAergic activity and may potentiate the sedative effects of drugs like lorazepam, diazepam, or zolpidem
- Diabetes medications: Some evidence suggests ashwagandha can lower blood glucose. If you take metformin, insulin, or sulfonylureas, this additive effect could increase hypoglycemia risk
- Blood pressure medications: Ashwagandha may modestly lower blood pressure. Combined with antihypertensives, this could cause excessive drops
The general principle: ashwagandha is pharmacologically active. It is not a "just a plant" that exists outside the rules of drug interaction. If you take prescription medications, discuss ashwagandha with your prescribing physician.
Who Should Not Take Ashwagandha
Based on the available evidence, the following groups should avoid ashwagandha or use it only under medical supervision:
- Pregnant or breastfeeding women: Ashwagandha has been classified as an abortifacient in traditional Ayurvedic medicine. Animal studies have shown reproductive effects at high doses. There are no adequate human safety studies in pregnancy. Do not take it.
- People with autoimmune diseases: Lupus, rheumatoid arthritis, multiple sclerosis, Hashimoto's thyroiditis. Ashwagandha's immune-modulating properties could exacerbate autoimmune flares.
- People with hyperthyroidism or Graves' disease: The thyroid-stimulating effect is directly contraindicated.
- People with liver disease or elevated liver enzymes: Given the documented hepatotoxicity case reports.
- People scheduled for surgery: Ashwagandha may affect blood pressure and interact with anesthesia. Discontinue at least two weeks before scheduled procedures, consistent with standard guidance for herbal supplements.
- People taking immunosuppressive medications: The immune-stimulating properties could undermine the therapeutic goal of immunosuppression.
KSM-66 vs Sensoril vs Generic: Does the Extract Matter for Safety?
The two most studied standardized extracts are KSM-66 (a full-spectrum root extract standardized to 5% withanolides) and Sensoril (a root and leaf extract standardized to 10% withanolides). The majority of clinical trial safety data comes from these two extracts. Generic, non-standardized ashwagandha powders have less predictable withanolide content, which introduces variability in both efficacy and safety.
From a safety standpoint, using a standardized extract (KSM-66 or Sensoril) at the studied dose gives you the most reliable safety data to reference. A generic powder at an unknown concentration is a less controlled exposure. This is not a guarantee of safety - the liver injury cases involved branded extracts in some instances - but standardization reduces one variable.
See our ashwagandha scorecard for product-level comparisons including which extract each product uses.
How Long Can You Take It?
Most clinical trials run 8-12 weeks. There is no published long-term safety data (6+ months) from randomized controlled trials. This does not mean long-term use is dangerous - it means we do not know with the same confidence that we know about short-term use.
Some practitioners recommend cycling ashwagandha (8 weeks on, 2-4 weeks off) to reduce the theoretical risk of receptor downregulation and to give the liver periodic breaks. This is a reasonable precaution based on general pharmacological principles, though it has not been specifically validated in ashwagandha trials. If you use ashwagandha long-term, periodic liver function testing is a sensible addition to your routine bloodwork.
FAQ
Can ashwagandha cause weight gain?
Some clinical trials have reported modest increases in body weight (1-2 kg over 8-12 weeks), but this appears to be primarily lean mass, particularly in studies combining ashwagandha with resistance training. Ashwagandha does not appear to increase fat mass. The cortisol-lowering effect could theoretically reduce stress-related overeating in some people, which might actually support weight management.
Does ashwagandha affect hormones in women?
Ashwagandha can affect thyroid hormones in both sexes, as discussed above. There is limited evidence that it may modestly affect testosterone and DHEA-S levels. A 2019 study found that ashwagandha improved sexual function scores in healthy women, with some hormonal changes noted. Women with hormone-sensitive conditions (certain breast cancers, endometriosis, PCOS) should consult their physician before use.
Can you take ashwagandha with caffeine?
There is no documented adverse interaction between ashwagandha and caffeine. In fact, the combination of ashwagandha's calming GABAergic effects with caffeine's stimulant properties is a common stack in the nootropic community. Some users find that ashwagandha reduces the jitteriness of caffeine while preserving the alertness. This is anecdotal, not clinically studied.
Is ashwagandha safe for teenagers?
There are no clinical trials of ashwagandha in adolescents. Given its hormonal effects (thyroid, potential testosterone/DHEA modulation), use in teenagers is not recommended without physician supervision. Hormonal systems in adolescents are still developing, and introducing exogenous modulators carries more risk than in adults with established hormonal baselines.
These statements have not been evaluated by the FDA. Dietary supplements are not intended to diagnose, treat, cure, or prevent any disease.