Ashwagandha: Benefits, Side Effects, and What the Research Actually Says

Suppi Research Team · February 28, 2026 · 11 min read

Ashwagandha has gone from an obscure Ayurvedic herb to one of the best-selling supplements in the US. It's everywhere—standalone capsules, nootropic stacks, sleep formulas, testosterone boosters. The marketing makes it sound like a miracle: lower stress, better sleep, more testosterone, less anxiety, improved exercise performance.

Some of that is actually supported by research. Some of it is exaggerated. And there are side effects that the marketing conveniently forgets to mention, including thyroid interference and rare but documented liver concerns.

Here's a straight read of the evidence.

What Ashwagandha Actually Is

Withania somnifera is a nightshade family plant native to India, the Middle East, and parts of Africa. The name "ashwagandha" translates to "smell of the horse," which refers both to its distinct odor and the traditional belief that it gives you the strength of a horse. The "somnifera" part means sleep-inducing.

The active compounds are withanolides—steroidal lactones. Different parts of the plant (root, leaf, fruit) contain different concentrations and types of withanolides, which is why extract type matters.

KSM-66 vs Sensoril: They're Not the Same

This distinction matters and most articles gloss over it. The two most studied and standardized extracts are quite different.

If you've tried ashwagandha and it either didn't work or made you too drowsy, you may have been using the wrong extract for your goals. KSM-66 is better for daytime energy and exercise. Sensoril is better for anxiety and nighttime use.

Benefit #1: Stress and Cortisol Reduction — Strong Evidence

This is the best-supported use. The landmark study is Chandrasekhar et al. (2012): 64 adults with chronic stress were given either 300mg KSM-66 twice daily or placebo for 60 days. Results:

• Serum cortisol dropped 27.9% vs 7.9% in placebo
• Perceived Stress Scale scores dropped significantly
• Sleep quality improved
• No serious adverse events

A 2019 systematic review by Lopresti et al. analyzed 5 RCTs with 400+ total participants and confirmed significant cortisol reduction and stress improvement across studies. The effect sizes were clinically meaningful, not just statistically significant.

A 2014 study using Sensoril (125mg and 250mg) found dose-dependent reductions in cortisol and improvements in stress and anxiety scores, with the higher dose showing more robust effects.

The mechanism appears to involve modulation of the HPA axis and GABAergic activity. It doesn't just mask stress—it appears to normalize the physiological stress response over time.

Benefit #2: Sleep Quality — Moderate Evidence

A 2019 double-blind RCT in Cureus gave 150 healthy subjects either 120mg of ashwagandha or placebo. The ashwagandha group showed significant improvements in sleep onset latency (how quickly they fell asleep) and sleep quality. The effect was strongest in subjects with pre-existing insomnia.

A 2020 meta-analysis pooled 5 RCTs and found ashwagandha improved overall sleep quality scores. Interestingly, the effect was stronger at doses above 600mg/day, and the sleep benefits were more pronounced in people who were also anxious or stressed.

This makes sense if the primary mechanism is cortisol and anxiety reduction. If you can't sleep because your mind is racing with stress, ashwagandha addresses the root cause rather than sedating you.

Benefit #3: Testosterone — Modest but Real

This is where the supplement marketing gets aggressive. The truth is more nuanced.

A 2019 RCT gave 43 overweight men (ages 40-70) 300mg of KSM-66 twice daily for 8 weeks. Results: testosterone increased by about 15% vs placebo, and DHEA-S also increased. But the absolute numbers matter—a 15% increase in a man with low-normal testosterone might take him from, say, 380 to 437 ng/dL. Helpful, but it's not going to turn someone into a bodybuilder.

A 2015 study in men doing resistance training found that KSM-66 at 300mg twice daily significantly increased testosterone compared to placebo, alongside improvements in muscle strength and recovery.

Importantly, the effects seem most pronounced in men who are stressed or have suboptimal testosterone. If your testosterone is already in the healthy range and your stress levels are low, don't expect dramatic changes.

Benefit #4: Exercise Performance — Moderate Evidence

A 2015 study found that 600mg/day of ashwagandha root extract (KSM-66) for 12 weeks significantly improved cardiorespiratory endurance (VO2max) and self-reported physical health in healthy, athletic adults.

Another 2015 study on resistance training found that KSM-66 users had greater increases in muscle strength (bench press and leg extension), greater muscle size, and better recovery (lower exercise-induced muscle damage) compared to placebo.

A small 2021 study found that ashwagandha improved anaerobic running capacity in elite athletes, though the sample size was limited.

The exercise effects are real but moderate. Don't expect creatine-level performance gains. Think of it as a 5-10% edge, mostly through better recovery and stress management.

Side Effects: What Actually Gets Reported

Common (generally mild)

• GI discomfort — Nausea, diarrhea, stomach pain. Usually dose-dependent. Taking with food helps. Reported in roughly 5-10% of clinical trial participants.
• Drowsiness — Particularly with Sensoril or high doses. This is a feature for nighttime use and a bug for daytime use.
• Headache — Occasional reports in clinical trials, usually mild and transient.

Thyroid effects — Important

This is the side effect most people don't know about. Ashwagandha can increase thyroid hormone levels. A 2018 study in the Journal of Alternative and Complementary Medicine found that 600mg/day of ashwagandha root extract for 8 weeks significantly increased T4 levels in subclinically hypothyroid patients.

For people with hypothyroidism, this might sound like a benefit. But it creates real problems:

• If you're on levothyroxine (Synthroid), ashwagandha could throw off your carefully titrated dose
• If you have hyperthyroidism or Graves' disease, ashwagandha could worsen it
• Unmonitored thyroid changes can affect heart rate, metabolism, and mood

If you take thyroid medication or have any thyroid condition, talk to your endocrinologist before using ashwagandha. This isn't a theoretical concern—case reports exist of clinically significant thyroid changes.

Liver concerns

Starting around 2020, case reports began surfacing of ashwagandha-associated liver injury. The Iceland Drug Safety Database documented multiple cases, and a 2023 review in the Hepatology Communications journal cataloged over 20 cases of clinically apparent liver injury attributed to ashwagandha, some requiring hospitalization.

Context matters: millions of people take ashwagandha and these cases number in the dozens. The absolute risk is very low. But it's not zero, and it's something you should be aware of, especially if you:

• Take other medications metabolized by the liver
• Consume alcohol regularly
• Have pre-existing liver conditions
• Take ashwagandha at doses above what's used in clinical trials

Other interactions

• Sedatives/benzodiazepines — May enhance sedation
• Immunosuppressants — Ashwagandha may stimulate immune activity, potentially counteracting immunosuppressive drugs
• Blood sugar medications — Some evidence of blood sugar-lowering effects; could enhance hypoglycemia risk

Dosage and Timing

Based on clinical trial data:

Most clinical trials run 8-12 weeks. Effects on stress and sleep tend to appear within 2-4 weeks. Testosterone and exercise effects take closer to 8 weeks.

Cycling isn't strictly necessary based on current evidence, but some practitioners recommend 8 weeks on, 2 weeks off as a precaution, partly because of the thyroid effects. There aren't long-term (1+ year) safety studies, so this conservative approach has some logic to it.

Who Should Skip Ashwagandha

• People with hyperthyroidism or autoimmune thyroid conditions
• People on thyroid medication (without doctor approval)
• Pregnant or breastfeeding women (insufficient safety data)
• People scheduled for surgery (potential immune and sedative effects)
• Anyone on immunosuppressants
• People with liver disease

Ashwagandha is a genuinely useful supplement for stress and possibly sleep, with emerging evidence for exercise and testosterone. But it's not risk-free, and the thyroid interaction in particular deserves more attention than it gets. Use a standardized extract (KSM-66 or Sensoril), stick to studied doses, and pay attention to how your body responds.