Ashwagandha for Endurance Athletes: Benefits and Risks

Ashwagandha is currently the best-selling adaptogen in the sports market — sold as a sleep aid, a stress blocker and a recovery booster in one capsule. The substance is not ineffective, but the evidence is far thinner than the packaging suggests: small groups, short durations, inconsistent extracts. This article separates what is proven from what is marketing, and names the risks that sales pages usually leave out.

What ashwagandha is and why it shows up in sport

Ashwagandha (Withania somnifera) is a plant native to India, Pakistan and parts of Africa, belonging to the nightshade family. The root is used, more rarely the leaf. The likely active compounds are withanolides — steroidal lactones whose content in standardised extracts ranges from 2.5 to 10 percent. That content decides whether you are taking 300 milligrams of an effective extract or 300 milligrams of root powder with largely unknown composition.

The term "adaptogen" comes from Soviet pharmacology of the 1940s and describes substances meant to increase resistance to stress without altering body functions. For endurance athletes this is attractive for two reasons. First, cortisol rises chronically with high training volume, and chronically elevated cortisol is considered a brake on recovery and adaptation. Second, sleep problems before races and during heavy blocks are widespread, and ashwagandha is one of the few plant substances with controlled sleep trials behind it.

The third reason is less scientific: ashwagandha is cheap, available without prescription and perceived as having no side effects. That is where the sobering part begins.

What the evidence actually shows

Research on ashwagandha in sport falls into four areas that are very differently supported.

• Sleep — best supported: Several randomised trials with 300 to 600 milligrams of root extract over six to ten weeks showed improved sleep efficiency and shorter time to fall asleep. Effects are moderate but consistent across studies. For endurance athletes this is the most indirectly relevant point, because sleep is the strongest recovery factor you can influence — see Sleep and endurance performance.
• Cortisol — plausible but overrated: Several studies found morning cortisol about 10 to 15 percent lower than placebo. Lower cortisol is not a performance goal in itself, though. Cortisol is a necessary part of training adaptation: it mobilises energy and triggers repair processes. Lowering it across the board may also lower the stimulus.
• Strength and muscle — small effects: Trials in strength trainees showed slightly larger gains in strength and lean mass over eight weeks than placebo, alongside 600 milligrams daily. For endurance athletes this is a side note: strength training belongs in every plan, but ashwagandha does not replace it.
• Endurance performance — thin: One frequently cited trial in recreational cyclists over eight weeks reported a 3 to 4 percent improvement in maximal oxygen uptake. That is the only reasonably solid endurance finding, and it comes from a small group without replication by other research groups. Turning it into a "+4 % VO2max" promise stretches the data massively.

Sample sizes matter here: many of these studies use 20 to 60 people, often untrained or recreationally active, frequently without verifying the withanolide content of the product. A rule from training science applies: an effect that appears in one small study and not in the next is probably not an effect. By comparison, for carbohydrate periodization or interval training there are dozens of studies with consistent results — with percentage points that actually show up on race day.

The cortisol myth: why "lowering the stress hormone" is not a training goal

Few supplements are sold on such a simple narrative as ashwagandha: cortisol is the stress hormone, stress is bad, so we lower cortisol. That narrative does not survive exercise physiology.

Cortisol has three core jobs in training. It mobilises glucose from the liver once a session runs longer than glycogen stores allow. It raises fat oxidation as an indirect consequence. And it is part of the signalling chain that initiates repair after the stimulus. A cortisol level that fails to rise during a hard session would not be a sign of fitness but of adrenal insufficiency — a medical problem, not a training state.

The real connection between chronically high cortisol and poor adaptation is more complex. It is not the absolute value that is problematic but a pattern: persistently elevated evening values, a flattened daily rhythm and insufficient decline overnight. That exact pattern emerges from too much volume, too little sleep and too many hard sessions at once. Ashwagandha can soften the symptom. It does not fix the cause — the cause is the plan.

In practice: if you show signs of overtraining, the first step is analysing your training load, not taking a capsule. Peakora works with TSS, CTL and TSB — training load, fitness and form — and these numbers show you earlier and more reliably than any blood marker whether you are overreaching. How those metrics relate is explained in CTL, ATL and TSB; the warning signs are collected in Overtraining: 10 warning signs.

Dose, extract quality and timing

The protocols used in studies are remarkably uniform — and that uniformity is itself a hint at how narrow the evidence base is:

• Dose: 300 to 600 milligrams of standardised root extract per day, usually split into two doses. Higher doses produced no additional benefit in trials, only more side effects.
• Duration: eight to twelve weeks. Effects on sleep and cortisol required at least six weeks. A single dose before a race does nothing — the substance has no acute effect.
• Standardisation: look for the declared withanolide content (typically 2.5 to 5 percent) plus the root as raw material. Leaf extracts are cheaper and less well studied.
• Timing: in the evening, 30 to 60 minutes before bed, if the goal is sleep quality. For other goals timing is secondary.

On quality, a realistic note: botanical extracts are among the most frequently adulterated raw materials in the sports nutrition market. Studies analysing commercial ashwagandha products regularly found products without detectable withanolides or with undeclared fillers. If you do not have a product with a batch number and certificate of analysis, test it yourself: good root extract smells earthy and bitter and tastes distinct; capsules that taste of nothing are often mostly maltodextrin.

A second practical point: ashwagandha is no substitute for magnesium or iron. A deficiency in magnesium or iron causes concrete performance losses and can be measured — the evidence there is unambiguous, and the order is always: rule out a deficiency first, then consider adaptogens.

The risks that sales pages leave out

Ashwagandha is considered well tolerated, but not by everyone. Three risk areas are documented and should be known before the first capsule.

• Liver: There are documented individual cases of liver injury up to drug-induced hepatitis, some requiring hospitalisation. Frequency is very low, but the association is better documented for ashwagandha than for most other botanical supplements. If your liver values are already outside the reference range or you drink alcohol regularly, leave it alone.
• Thyroid: Several studies show a rise in T3 and T4 on ashwagandha. With a healthy thyroid this usually stays within the reference range. With hypothyroidism, Hashimoto's thyroiditis or ongoing hormone therapy, taking it without medical supervision is a bad idea.
• Stomach and circulation: The most common side effects in trials are gastrointestinal upset, nausea and drowsiness. Rare reports describe drops in blood pressure — relevant if yours is low to begin with.

Two groups should not take it at all: pregnant and breastfeeding women. The evidence is insufficient and there are hints of uterine effects. And anyone on medication should check for interactions first — relevant are thyroid medication, immunosuppressants, sedatives and blood-pressure drugs.

Where ashwagandha fits into a training week

If you want to try it, do so in a clearly limited window with a clearly defined question. The most sensible use case is not race performance but sleep quality during high-load phases — the weeks with the biggest volumes, or after late sessions that leave you unable to fall asleep.

• Sensible scenario: eight weeks in a build phase with high volume, 300 milligrams in the evening, combined with a consistent bedtime. Track sleep duration, resting heart rate and perceived exertion (RPE).
• Pointless scenario: taking it on race day or in race week. There is no acute effect, and you only risk an unknown reaction.
• What to measure: not how you feel in the morning but measurable values. Resting heart rate, HRV trend, sleep efficiency from your watch, and above all your pace at the same heart rate over several weeks.
• Stop criteria: stomach upset, persistent fatigue, yellowing of eyes or skin, dark urine. In the last case stop immediately and have liver values checked.

Realistically, ashwagandha is an option for a narrow group: endurance athletes with high volume and genuinely poor sleep quality in loading phases who already have their load management under control. For everyone else the benefit is too small for the risk.

What delivers more instead

The honest ranking of recovery factors is unspectacular and backed by dozens of studies. Work through it in order and you will rarely need the later stages.

• Sleep duration: seven to nine hours. The single biggest factor. No supplement comes close.
• Calories and carbohydrates: under-fuelling is the most common cause of stalled adaptation — see RED-S and energy availability.
• Load management: raising weekly volume by more than ten percent is a risk factor, not proof of progress. How to progress properly is covered in Increasing training volume.
• Protein: 1.6 to 2.2 grams per kilogram of body weight per day — see Protein for endurance athletes.
• Only then: micronutrients with a proven deficiency, and adaptogens as the last stage.

Anyone who implements these five points will notice little from ashwagandha — and that is not an argument against the substance but for the order. A supplement that improves an already good state by one or two percent is uninteresting while three hours of sleep per night are missing and calorie intake sits below expenditure.

Frequently asked questions about ashwagandha