Ashwagandha Side Effects UK: Thyroid, Sedation, Dosing

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Ashwagandha's most common side effects are mild stomach upset and drowsiness at higher doses, but the two issues worth taking seriously are its effect on thyroid hormone levels and a small, growing set of liver injury case reports linked to standardised root extracts. Most healthy adults tolerate 300-600mg of extract a day without any of this, but "most people" is not the same as "everyone," and a few groups need to check with a doctor before starting.

Ashwagandha (Withania somnifera) has become one of the best-selling herbal supplements in the UK, and our own best ashwagandha supplement guide covers why: a genuinely strong clinical evidence base for stress, sleep, and hormonal support. That popularity is exactly why the safety picture deserves its own honest look. Ashwagandha is not a vitamin with a wide margin for error; it's a pharmacologically active herb that measurably changes hormone levels in some people, and the growing volume of use has surfaced adverse event reports that weren't visible when the ingredient was more niche.

This guide sets out what the clinical trial data, published case reports, and current UK regulatory review actually say, rather than repeating the blanket "generally safe" line found on most retail product pages. None of this is a reason to avoid ashwagandha outright. It's a reason to take the dose, the timing, and a handful of specific contraindications seriously.

Key Takeaways (TL;DR)

• Thyroid hormones can rise: clinical trials show ashwagandha lowering TSH and raising T3/T4, useful in subclinical hypothyroidism but risky if you're already on thyroid medication.
• Sedation is dose-linked: drowsiness is more likely at doses of 600mg/day and above, especially taken alongside sedative medication or alcohol.
• GI upset is the most common complaint: nausea and loose stools occur mainly on an empty stomach and usually resolve when taken with food.
• Liver injury is rare but documented: published case series describe cholestatic hepatitis linked to ashwagandha products, occasionally severe in people with existing liver disease.
• No UK safe upper limit exists yet: the Food Standards Agency's Committee on Toxicity has been reviewing ashwagandha since 2024, with no published conclusion as of this guide.
• Avoid in pregnancy and breastfeeding: evidence is too limited to call it safe, and it should be paused before use is confirmed appropriate.
• Most RCTs use 250-600mg/day of standardised extract for 8-12 weeks; going well beyond that range without medical advice isn't supported by the evidence.

The Thyroid Interaction: Why Ashwagandha Can Raise T3 and T4

Ashwagandha's effect on thyroid hormones is the best-documented and most clinically relevant safety issue on this list, because it cuts both ways. In people with subclinical hypothyroidism, a randomised, double-blind, placebo-controlled trial found that 600mg/day of ashwagandha root extract (split into two 300mg doses) over eight weeks significantly lowered TSH while raising both T3 and T4 compared with placebo, changes the researchers considered clinically meaningful.¹ For someone with an underactive thyroid who hasn't yet been diagnosed or treated, that's a plausible benefit.

The same mechanism becomes a problem in the opposite direction. Ashwagandha appears to stimulate the hypothalamic-pituitary-thyroid axis, and in people who don't need more thyroid hormone, that stimulation can tip them into biochemical hyperthyroidism. A published case describes a previously healthy man who developed painless thyroiditis, with classic thyrotoxicosis symptoms including fatigue, night sweats, weight loss, and palpitations, after two months of regular ashwagandha use; his symptoms and thyroid markers normalised once he stopped taking it.² Several similar reports exist in the literature.

The practical implication is straightforward: if you take levothyroxine or any other thyroid medication, or have a diagnosed thyroid condition of any kind, don't start ashwagandha without telling your GP or endocrinologist first. The dose that helps someone with an untreated underactive thyroid can push someone already on medication, or with an autoimmune thyroid condition like Hashimoto's or Graves', out of a stable range. This isn't a reason to panic if you've already been taking it without noticing anything, but it is a reason for a blood test if you're on thyroid treatment and considering adding it.

Sedation and Drowsiness: When Higher Doses Slow You Down

Ashwagandha isn't classified as a sedative in the way melatonin or valerian root are, but central nervous system depression has been observed in both animal models and human users, particularly at higher doses. A systematic review and meta-analysis of five randomised trials found that ashwagandha's effect on sleep was strongest, and most consistent, at doses of 600mg/day or more, especially in people who already had diagnosed insomnia; lower doses aimed purely at daytime stress reduction showed a smaller sleep effect.³ That's good news if better sleep is your goal and you're dosing in the evening. It's less good if you're taking a high dose in the morning expecting only stress relief and end up drowsy by mid-afternoon.

The overlap with prescribed sedatives, sleep medication, and alcohol is where this matters most. Combining ashwagandha with benzodiazepines, z-drugs like zopiclone, or other CNS depressants can compound the drowsiness rather than simply adding two mild effects together. If you're already on a prescribed sleep aid or anti-anxiety medication, mention ashwagandha to whoever prescribes it before combining the two, even though ashwagandha itself is available without a prescription.

For most people not on sedating medication, the practical fix is simple: take it in the evening if you notice drowsiness, and don't drive or operate machinery shortly after a new, higher dose until you know how it affects you personally. This is standard advice for any new supplement with a CNS-active ingredient, and ashwagandha is no exception.

Gastrointestinal Side Effects: Nausea, Loose Stools, and Timing

The single most commonly reported side effect across ashwagandha's clinical trial base isn't thyroid or liver related at all, it's the gut. Nausea, loose stools, stomach cramping, and mild digestive discomfort show up repeatedly in trial safety data, particularly at the higher end of the studied dose range and particularly when the supplement is taken on an empty stomach.⁴ The withanolides responsible for ashwagandha's activity appear to have a direct irritant effect on the gut lining in some people, which is a mechanical issue rather than a systemic one.

The fix is almost always straightforward: take ashwagandha with food rather than first thing in the morning on an empty stomach. Most people who report GI symptoms find they resolve within a few days of switching the timing, without needing to stop the supplement altogether. If cramping or loose stools persist beyond a week or two despite taking it with meals, that's a reasonable point to drop the dose or stop rather than push through.

This category of side effect is mild and self-limiting for the overwhelming majority of users, which is exactly why it gets the least attention compared with thyroid or liver effects. But it's also the one you're statistically most likely to actually experience, so it's worth planning around from day one rather than treating as a surprise.

Liver Safety: What the Case Reports Actually Show

This is the newer and less settled part of ashwagandha's safety story. Since the first published report in 2017, a growing number of case reports and small case series have described liver injury temporally linked to ashwagandha-containing supplements. A case series and literature review from India examined patients with ashwagandha-related liver injury between 2019 and 2022, finding cholestatic hepatitis as the most common presentation; among patients with pre-existing chronic liver disease, three progressed to acute-on-chronic liver failure, a serious and occasionally fatal complication.⁵ Other individual case reports describe similar patterns in people without known prior liver disease, generally resolving after the supplement was stopped, though not universally.

It's worth being precise about what this evidence does and doesn't show. These are case reports and small case series, the weakest form of clinical evidence for establishing causation, not the randomised trials that back ashwagandha's benefits. The exact ingredient responsible hasn't been pinned down; withanolides are the leading suspect, but contamination, individual susceptibility, and interactions with other medications or supplements taken at the same time can't be ruled out in most published cases. Against that, the tens of millions of doses taken globally without any reported liver problem suggest the absolute risk to a healthy person is low. Both things are true at once: the events are rare, and they are real enough that regulators are paying attention.

That regulatory attention is genuinely live in the UK right now. The Food Standards Agency opened a call for evidence on ashwagandha's safety in food supplements in 2024, specifically flagging thyroid effects, low blood sugar, and potential liver toxicity as areas of concern, and referred the evidence to the Committee on Toxicity for a formal risk assessment.⁶ As of this guide, that review is still in progress, a second draft statement was discussed at the Committee's May 2026 meeting, and no safe upper limit or restriction has yet been published. There is currently no legal cap on ashwagandha dose in UK food supplements, but that is a gap in regulation rather than a clean bill of health, and it's worth checking for an update if you're a long-term daily user.

If you have any pre-existing liver condition, including fatty liver disease, hepatitis, or a history of elevated liver enzymes, talk to your GP before starting ashwagandha. For everyone else, the practical advice is to watch for jaundice (yellowing skin or eyes), unusually dark urine, persistent nausea, or right-upper-abdominal pain in the weeks after starting, and stop and seek medical advice if any of these appear.

Who Should Talk to a Doctor Before Taking Ashwagandha

A handful of groups should get medical advice before starting ashwagandha rather than simply trying it and seeing what happens:

• Anyone on thyroid medication or with a diagnosed thyroid condition, given the documented ability to shift TSH, T3, and T4.¹
• Anyone pregnant, breastfeeding, or trying to conceive. Evidence in pregnancy is too limited to call ashwagandha safe, and it should be avoided rather than assumed harmless.
• Anyone with an autoimmune condition, particularly autoimmune thyroid disease, since ashwagandha's immune-modulating effects are not well characterised in this group.
• Anyone with existing liver disease, given the case reports of more severe outcomes in people with pre-existing hepatic conditions.⁵
• Anyone taking sedatives, sleep medication, or being treated for diabetes, because of the sedation overlap and ashwagandha's reported blood-sugar-lowering effect, which can compound with glucose-lowering medication.
• Anyone about to undergo surgery, since the same blood-sugar and sedative effects are relevant to anaesthesia and should be flagged to a surgical team in advance.

None of this means ashwagandha is dangerous for the average healthy adult without these conditions. It means these specific groups face a different risk calculation, and "ask your GP first" is genuinely the right answer rather than a boilerplate disclaimer.

Safe Dosing Limits: How Much Is Too Much

There's no single officially mandated maximum dose for ashwagandha in the UK, which makes the clinical trial literature the best available guide. The overwhelming majority of randomised controlled trials, including the ones that established ashwagandha's stress, sleep, and hormonal benefits, used between 250mg and 600mg per day of standardised root extract (typically 5% withanolides), taken for 8 to 12 weeks.⁷ Trials specifically targeting insomnia tended to use doses at the higher end of that range, around 600mg/day, while general stress-reduction studies often used less.

Products that advertise very high "root equivalent" figures, several grams per capsule, aren't necessarily exceeding this range; they're usually describing a concentrated extract in terms of how much raw root powder would be needed to match it, not asking you to consume grams of extract itself. Our ashwagandha buying guide covers how to read these labels properly so you're comparing genuine extract dose rather than marketing figures. What the evidence doesn't support is stacking multiple ashwagandha products together, or taking several times a labelled serving in the hope of amplifying the effect; none of the safety or efficacy data covers that territory, and it's the dosing pattern most associated with the sedation and GI complaints above.

A sensible approach for most healthy adults: start at the lower end (around 250-300mg/day of a 5% withanolide extract), give it two to four weeks to assess tolerance, and only consider the upper end of the studied range if you're specifically targeting sleep and tolerating it well. Continuous daily use beyond the roughly three-to-six-month window studied in most trials hasn't been well characterised either way, so periodic breaks are a reasonable precaution even in the absence of a specific rule requiring them.

FAQ

Can I take ashwagandha with levothyroxine or other thyroid medication?

Speak to your GP first. Clinical trials show ashwagandha can lower TSH and raise T3 and T4, which is why it's shown some benefit in subclinical hypothyroidism, but the same mechanism could push someone already on thyroid medication out of a stable range.¹ Case reports also describe thyrotoxicosis developing in previously healthy people after regular ashwagandha use.² If you're on thyroid treatment, a follow-up blood test after starting is a reasonable precaution.

Does ashwagandha make you drowsy during the day?

It can, particularly at doses of 600mg/day and above, which is also the dose range most associated with sleep benefits in trials.³ If you notice daytime drowsiness, moving your dose to the evening usually resolves it. Combining ashwagandha with sedative medication, sleep aids, or alcohol can make drowsiness more pronounced.

Is ashwagandha bad for your liver?

For most healthy people, no clear pattern of liver harm has emerged despite widespread use, but a real and growing set of published case reports links ashwagandha to liver injury, ranging from mild enzyme elevations to, rarely, acute liver failure in people with pre-existing liver disease.⁵ Anyone with existing liver problems should check with a doctor first, and anyone new to ashwagandha should watch for jaundice, dark urine, or persistent nausea in the first few weeks.

What's the maximum safe dose of ashwagandha per day?

There's no UK-mandated legal limit, but almost all the clinical evidence for both benefit and safety sits within 250-600mg/day of standardised root extract for 8-12 weeks.⁷ Going well beyond that range, or stacking multiple ashwagandha products, isn't supported by trial data and is the pattern most associated with sedation and stomach upset.

Can pregnant women take ashwagandha?

No, ashwagandha isn't recommended during pregnancy or while breastfeeding. The evidence in pregnant populations is too limited to establish safety, and it should be paused rather than assumed harmless until a doctor confirms otherwise.

Is ashwagandha currently under regulatory review in the UK?

Yes. The Food Standards Agency opened a call for evidence on ashwagandha's safety in 2024, citing thyroid, blood sugar, and liver concerns, and referred it to the Committee on Toxicity for a formal risk assessment.⁶ That review is ongoing, with no published safe limit or restriction as of this guide, so it's worth checking for updates if you're a regular long-term user.

The Bottom Line on Ashwagandha Safety

Ashwagandha remains one of the best-evidenced adaptogens available, and none of the above changes that for the average healthy adult taking a sensible dose. The two issues that deserve real attention are the thyroid hormone effect, which matters most if you're already on thyroid medication or have a diagnosed thyroid condition, and the liver case reports, which are rare but real enough that current UK regulators are actively reviewing them. Sedation and GI upset are common but minor, and both usually resolve with a lower dose or a change in timing.

If you fall into one of the groups flagged above, pregnant, on thyroid medication, managing liver disease, or taking sedatives, talk to a GP before starting rather than after something goes wrong. For everyone else, staying within the 250-600mg/day range the clinical trials actually used, taking it with food, and paying attention to how you personally respond remains the sensible way to use a genuinely effective supplement. For product-specific guidance on choosing a quality extract, see our full ashwagandha buying guide.