The Good News First
Ashwagandha has an excellent safety profile. Multiple randomised clinical trials have used it for up to 12 weeks without significant adverse effects. A systematic review of 69 studies concluded ashwagandha is "safe and well tolerated at the doses used in clinical trials." Serious adverse events are rare and typically associated with very high doses or specific pre-existing conditions.
That said, "natural" does not mean side-effect-free. Here's an honest breakdown of what the evidence shows.
Common Side Effects
Digestive Discomfort
The most commonly reported side effect in clinical trials is mild gastrointestinal upset — nausea, loose stools, or stomach cramps — particularly when taken on an empty stomach. This affects a minority of users and typically resolves by taking ashwagandha with food or reducing the dose. Switching to a capsule form (rather than powder mixed in liquid) can also help.
Drowsiness
Ashwagandha's cortisol-lowering and GABAergic effects can cause drowsiness in some people, particularly at higher doses (600mg+). This is why some people take it in the evening. If you experience drowsiness, shift your dose to before bed — this simultaneously supports sleep and avoids daytime impairment.
Headache
Some users report mild headaches, particularly in the initial weeks. This often resolves as the body adapts. Starting at a lower dose (150mg) and building up can reduce this.
Serious Considerations
Thyroid Function
Ashwagandha can increase thyroid hormone levels (T3 and T4). For most people this is beneficial or neutral. However, if you have hyperthyroidism or are on thyroid medication, this effect could be problematic. A 2019 study found ashwagandha significantly increased T3 and T4 levels — anyone on levothyroxine or with thyroid disease should consult their GP before using ashwagandha.
Pregnancy
Ashwagandha is contraindicated in pregnancy. Animal studies suggest high doses may induce uterine contractions (abortifacient properties). While human data is limited, the precautionary recommendation is to avoid it during pregnancy. It is considered safe during breastfeeding at normal doses, though data is limited.
Autoimmune Conditions
Ashwagandha modulates immune function — potentially stimulating immune activity. For people with autoimmune conditions (rheumatoid arthritis, lupus, multiple sclerosis) or on immunosuppressant drugs, this effect could theoretically worsen the condition or interact with medication. Consult a specialist if this applies to you.
Liver Health
Rare cases of drug-induced liver injury have been reported in people taking ashwagandha supplements, typically with high doses or prolonged use beyond 12 weeks. These cases are very rare and often involved products of uncertain quality or dose. Using standardised extracts (KSM-66 or Sensoril) from reputable brands at recommended doses significantly reduces this risk. Those with liver conditions should consult their GP.
Surgery
Ashwagandha may slow the central nervous system. Stop taking it at least 2 weeks before planned surgery, as it may interact with anaesthesia.
Drug Interactions
Ashwagandha may interact with:
- Thyroid medications (levothyroxine, methimazole) — may potentiate effects
- Immunosuppressants — may counteract drug effects
- Sedatives and CNS depressants — additive sedative effects
- Diabetes medications — ashwagandha has mild blood glucose-lowering effects that may enhance medication
- Blood pressure medications — may have additive effect
Who Should Avoid Ashwagandha
- Pregnant women
- People with hyperthyroidism
- People with active liver disease
- Those on immunosuppressants
- People with nightshade (Solanaceae) plant allergies (ashwagandha is in this family)
How to Minimise Side Effects
- Always take with food
- Start at a lower dose (150–300mg) and build up
- Use standardised extracts (KSM-66, Sensoril) from reputable brands
- Don't exceed 600mg/day without clinical guidance
- Consider cycling — 8–12 weeks on, 2–4 weeks off
Bottom Line
Ashwagandha is genuinely one of the safest evidence-backed supplements available, with an excellent tolerance profile in clinical trials. The most common side effects (GI upset, drowsiness) are mild and manageable. The serious considerations — thyroid effects, pregnancy, liver injury — are real but rare, primarily affect specific populations, and are largely preventable with quality products at appropriate doses. Always check with your GP if you have pre-existing conditions or are on medication.