Bladderwrack (Fucus vesiculosus) is a brown seaweed from rocky North Atlantic shores, named for the air-filled bladders that float it upright in the water. It naturally contains iodine, in amounts that vary with season and site, so anyone with a thyroid condition or taking thyroid medicine, lithium or amiodarone should ask a doctor first, and it is not for pregnancy, breastfeeding or under-18s.
Key takeaways
- A brown seaweed, not sea moss. Irish sea moss is a different, red seaweed.
- It naturally contains iodine, and the amount varies. It changes with species, site and season.
- No authorised health claims. The European Medicines Agency (EMA) accepts bladderwrack medicines on long-standing use alone. That covers registered herbal medicines, not food supplements.
- Its iodine is behind most of the cautions. Thyroid condition, or taking thyroid medicine, lithium or amiodarone? Speak to your doctor first. Do not combine it with kelp, iodine or another seaweed supplement.
- Not for pregnancy, breastfeeding or under-18s. Our Sea Moss Gummies also contain burdock root, so they are not for anyone allergic to daisy-family plants.
From us: Our Sea Moss Gummies contain 2,000mg of bladderwrack per two gummies, alongside 3,200mg of Irish sea moss and 480mg of burdock root (5,680mg in total). Vegan, £13.99 for 60 gummies (30 days), third-party lab tested.
Bladderwrack at a glance
| Latin name | Fucus vesiculosus L., a brown seaweed in the wrack family (Fucaceae) |
| Also known as | Bladder wrack, fucus. Some herbal sources call it kelp |
| Part used | The whole seaweed (the thallus), dried |
| Where it grows | Rocky shores of the North Atlantic and the Pacific |
| Traditional use | A European herbal medicine since at least the 1600s, mostly because of its iodine. Also used in cosmetics |
| In our gummies | 2,000mg of bladderwrack per two gummies, in Sea Moss Gummies |
| UK status | A food supplement ingredient with no authorised health claims. The EMA's traditional herbal monograph (2014, unchanged after a 2022 review) covers registered herbal medicines, not food supplements |
What is bladderwrack?
Bladderwrack grows 20 to 100cm long on rocky shores, with flat, forked fronds and air-filled bladders that float it upright in the water.
The EMA's assessment dates its use in European herbal medicine to at least the 1600s. Iodine itself was discovered in seaweed ash in 1811, by the Paris saltpetre maker Bernard Courtois.
Besides iodine, dried bladderwrack is about 12% alginic acid and 15% polyphenols, mostly phlorotannins, with a complex carbohydrate called fucoidan and minerals such as potassium, calcium and magnesium.
Bladderwrack, iodine and the thyroid
Your thyroid uses iodine to make its hormones. The NHS says adults need 140 micrograms a day. The upper level for adults, set by the EU's Scientific Committee on Food, is 600 micrograms a day from all sources.
In a study of 11 seaweeds from two Irish sites, brown wracks such as bladderwrack typically held about ten times more iodine than red seaweeds, true kelps (Laminaria) about ten times more again, and iodine was usually higher in winter than in summer.
The EMA concludes that iodine-containing bladderwrack preparations "can influence thyroid function". Too much iodine can make the thyroid overactive or underactive, and people with a thyroid condition, older people, and unborn and newborn babies are at higher risk. The EMA caps the iodine from registered bladderwrack medicines at 400 micrograms a day, and its assessment recommends limiting how long they are taken and checking thyroid function regularly, for example every four months. Taking any seaweed product for months? Ask your GP whether a thyroid blood test is worth doing.
Our bladderwrack and sea moss naturally contain iodine, and the amount varies from harvest to harvest, so we do not print a fixed iodine figure per gummy. Do not combine our Sea Moss Gummies with kelp, iodine or another seaweed supplement. More in Sea Moss and Thyroid Function: The Iodine Connection.
What has been studied?
The EMA notes that much bladderwrack research is in the lab and in animals. In people, we found a handful of small, short trials of bladderwrack on its own, mostly of extracts. Two examples:
- Australia, 2018. 38 healthy adults each took three single doses on separate days, double-blind: 500mg or 2,000mg of a polyphenol-rich bladderwrack extract, or a placebo, before eating white bread. Researchers measured blood sugar and insulin. Supported by a government scholarship and the extract's maker.
- The Netherlands, 2024. A five-week pilot trial gave 36 adults with type 2 diabetes 5g a day of dried bladderwrack (ten people), another brown seaweed or a little nori as a control, double-blind, alongside their usual treatment. Researchers tracked blood sugar. Funded by Health Holland; suppliers donated the seaweeds.
None of the trials we found tested bladderwrack with sea moss or tested our gummies. That is too little to support a health claim. The EMA's public summary says there is "insufficient evidence from clinical trials", and its 2022 review found "the quality of the studies is low".
How much do people take?
Bladderwrack is not a vitamin or mineral, so it has no Nutrient Reference Value (NRV), and no official upper level has been set for bladderwrack itself.
- In trials: single doses of 500mg or 2,000mg of an extract, or 5g a day of dried seaweed for five weeks.
- In registered herbal medicines: the EMA caps the iodine from bladderwrack at 400 micrograms a day and does not recommend use under 18.
Our Sea Moss Gummies contain 2,000mg of bladderwrack per two gummies, taken any time, with or without food, and no more than two a day. No trial has used our gummies, and extracts, dried seaweed and gummies are made differently, so their weights cannot be compared like for like. Their iodine depends on the seaweed, harvest and processing, not on weight alone.
Side effects and who should avoid it
Thyroid changes. If you notice a fast or irregular heartbeat, trembling, anxiety, unusual tiredness, feeling cold or weight gain after starting a seaweed product, stop and see your GP: the NHS lists these among the signs of an overactive or underactive thyroid.
Heavy metals. Seaweeds take up arsenic and heavy metals from the water. The EMA notes that bladderwrack from polluted water may carry enough to damage the kidneys, and says the quality of bladderwrack preparations "should be carefully controlled".
Menstrual cycle. A 2004 report on three women taking bladderwrack found their menstrual cycles lengthened by 5.5 to 14 days. Three people with no comparison group cannot show cause, but the EMA includes this report in the safety section of its bladderwrack monograph, so if your cycle changes, tell your GP.
Allergy. Burdock root, also in our gummies, is in the daisy family, and the EMA records anaphylaxis, a severe allergic reaction, with it. If your lips, mouth or throat suddenly swell, or you struggle to breathe, call 999.
If a new symptom appears, stop and speak to your GP or a pharmacist.
Who should not take our Sea Moss Gummies, or should ask first:
- Anyone pregnant or breastfeeding. Do not take them. The EMA does not recommend bladderwrack in pregnancy or breastfeeding, and LactMed, the US drugs and breastfeeding database, notes that iodine passes into breast milk.
- Anyone under 18. Our gummies are for adults.
- Anyone allergic to seaweed, to daisy-family plants or to any ingredient on the label. Do not take them.
- Anyone with a thyroid condition, high blood pressure, heart or kidney disease or anaemia, or due radioactive iodine treatment. Speak to your doctor first.
- Anyone taking thyroid medicine, lithium, amiodarone or another iodine-containing medicine, blood thinners or blood pressure medicines. Speak to your doctor first.
- Anyone taking diuretics (water tablets) or diabetes medicine. Ask your GP or pharmacist first.
- Anyone already taking kelp, iodine or another seaweed supplement. Do not combine them.
Food supplements are not a substitute for a varied diet and a healthy lifestyle.
Bladderwrack and medicines
- Thyroid medicines and iodine. The EMA monograph says bladderwrack medicines should not be taken with iodine preparations, thyroid medicines or other bladderwrack products.
- Lithium. A 2011 Spanish report described a 60-year-old man on lithium whose thyroid became overactive while he took a herbal laxative containing bladderwrack.
- Amiodarone. France's food safety agency, ANSES, notes that dietary iodine can change how this heart rhythm medicine affects the thyroid.
- Blood thinners and blood pressure medicines. The EMA's assessment lists possible interactions with both, so speak to your doctor first.
Irish sea moss + bladderwrack + burdock root
Sea Moss Gummies
£13.99 for 60 gummies, a 30-day supply. Strawberry or Apple flavour. Free UK delivery.
List prices, before any discount code, correct on 1 November 2026.
Frequently asked questions
What are the benefits of bladderwrack?
Bladderwrack has no authorised health claims of its own in the UK. We found a handful of small, short human trials of it on its own, mostly of extracts, and none tested it with sea moss or tested our gummies. That is too little to support a health claim, and the European Medicines Agency says there is "insufficient evidence from clinical trials" for bladderwrack medicines.
What are the side effects of bladderwrack?
The best documented risks come from its iodine, which can make the thyroid overactive or underactive, and from arsenic and heavy metals that seaweeds take up from the water. It is not recommended in pregnancy, while breastfeeding or under 18, or for anyone allergic to seaweed. Anyone with a thyroid condition, high blood pressure, heart or kidney disease or anaemia, or taking thyroid medicine, lithium, amiodarone, blood thinners or blood pressure medicines, should speak to a doctor first. Do not combine it with kelp, iodine or another seaweed supplement. If a new symptom appears, stop and speak to a GP or pharmacist.
How much iodine is in bladderwrack?
It varies with species, site and season. In a study of 11 seaweeds from two Irish sites, brown wracks such as bladderwrack typically held about ten times more iodine than red seaweeds, and iodine was usually higher in winter than in summer. The bladderwrack and sea moss in our Sea Moss Gummies naturally contain iodine, and the amount varies from harvest to harvest, so we do not print a fixed iodine figure per gummy. Do not combine them with kelp, iodine or another seaweed supplement. They are not for pregnancy, breastfeeding or under-18s, and with a thyroid condition or thyroid medicine, speak to your doctor first.
Is bladderwrack good for your thyroid?
Bladderwrack has no authorised health claims of its own, and the European Medicines Agency says iodine-containing bladderwrack preparations "can influence thyroid function". Too much iodine can make the thyroid overactive or underactive, and people with a thyroid condition are at higher risk. The agency advises against taking bladderwrack medicines with iodine preparations or thyroid medicines, and does not recommend them in pregnancy, while breastfeeding or under 18. If you have a thyroid condition, are due radioactive iodine treatment, or take thyroid medicine, lithium or amiodarone, speak to your doctor before taking any bladderwrack product, and do not combine it with kelp, iodine or another seaweed supplement.
What is the difference between bladderwrack and sea moss?
Bladderwrack (Fucus vesiculosus) is a brown seaweed with air bladders. Irish sea moss is usually Chondrus crispus, a red seaweed also called carrageen. In an Irish study, brown wracks such as bladderwrack typically held about ten times more iodine than red seaweeds. Both carry iodine, so with a thyroid condition or thyroid medicine, speak to a doctor before taking either, and neither is for pregnancy, breastfeeding or under-18s.
References
- European Medicines Agency, HMPC. "Community herbal monograph on Fucus vesiculosus L., thallus." EMA/HMPC/313674/2012, 6 May 2014.
- European Medicines Agency, HMPC. "Assessment report on Fucus vesiculosus L., thallus." EMA/HMPC/313675/2012, 6 May 2014.
- European Medicines Agency, HMPC. "Addendum to Assessment report on Fucus vesiculosus L., thallus." EMA/HMPC/681054/2021, 26 January 2022.
- European Medicines Agency. "Herbal medicine: summary for the public. Bladderwrack." EMA/661584/2016, 22 November 2016. ema.europa.eu.
- Royal Society of Chemistry. "Iodine." Periodic Table. rsc.org.
- NHS. "Vitamins and minerals: Iodine." Page last reviewed 3 August 2020. nhs.uk.
- Scientific Committee on Food. "Opinion of the Scientific Committee on Food on the Tolerable Upper Intake Level of Iodine." European Commission, 2002.
- Nitschke U et al. "Variability in iodine in temperate seaweeds and iodine accumulation kinetics of Fucus vesiculosus and Laminaria digitata (Phaeophyceae, Ochrophyta)." J Phycol, 2018. doi:10.1111/jpy.12606
- Leung AM, Braverman LE. "Consequences of excess iodine." Nat Rev Endocrinol, 2014. doi:10.1038/nrendo.2013.251
- Murray M et al. "The Impact of a Single Dose of a Polyphenol-Rich Seaweed Extract on Postprandial Glycaemic Control in Healthy Adults: A Randomised Cross-Over Trial." Nutrients, 2018. doi:10.3390/nu10030270
- Geurts KAM et al. "The Effect of Sargassum fusiforme and Fucus vesiculosus on Continuous Glucose Levels in Overweight Patients with Type 2 Diabetes Mellitus: A Feasibility Randomized, Double-Blind, Placebo-Controlled Trial." Nutrients, 2024. doi:10.3390/nu16121837
- NHS. "Overactive thyroid (hyperthyroidism): Symptoms." Page last reviewed 11 April 2023. nhs.uk.
- NHS. "Underactive thyroid (hypothyroidism)." Page last reviewed 28 April 2025. nhs.uk.
- Skibola CF. "The effect of Fucus vesiculosus, an edible brown seaweed, upon menstrual cycle length and hormonal status in three pre-menopausal women: a case report." BMC Complement Altern Med, 2004. doi:10.1186/1472-6882-4-10
- European Medicines Agency, HMPC. "Community herbal monograph on Arctium lappa L., radix." EMA/HMPC/246763/2009, 16 September 2010.
- NHS. "Anaphylaxis." Page last reviewed 21 June 2023. nhs.uk.
- Drugs and Lactation Database (LactMed). "Seaweed." National Institute of Child Health and Human Development, updated 15 October 2024. ncbi.nlm.nih.gov.
- Arbaizar B, Llorca J. "Fucus vesiculosus induced hyperthyroidism in a patient undergoing concomitant treatment with lithium." Actas Esp Psiquiatr, 2011 (article in Spanish). PMID 22127913.
- ANSES. "Opinion of the French Agency for Food, Environmental and Occupational Health & Safety on the risk of excess iodine intake from the consumption of seaweed in foodstuffs." Request No 2017-SA-0086, 25 June 2018. anses.fr.
Food supplements are not intended to treat, cure or prevent any disease. If you are pregnant, breastfeeding, taking medication, or have a medical condition, consult a healthcare professional before use.
