Bladderwrack (Fucus vesiculosus): Uses, Evidence and Safety

Safety first: iodine, the thyroid and arsenic

Bladderwrack is a seaweed, and two problems come with it. First, it is rich in iodine, and the EU's herbal committee (HMPC) concluded that taking iodine-containing bladderwrack products can influence thyroid function. [4] In a small 4-week trial of 36 healthy adults, kelp capsules (a related seaweed supplement) raised thyroid-stimulating hormone (TSH), a sign the thyroid was being pushed. [4] Second, seaweed can take up heavy metals, so the quality of the product matters. The EU assessment says bladderwrack grown in a contaminated environment, for example with arsenic or heavy metals, can lead people to swallow these toxic substances, and it lists a case of arsenic poisoning linked to a kelp supplement. [4][5] Memorial Sloan Kettering (MSK) also warns that bladderwrack from polluted waters may contain arsenic, cadmium and mercury and may harm the kidneys. [1]

The EU assessment lists these as reasons not to use it: an overactive thyroid, Graves disease, Hashimoto thyroiditis, a partly removed thyroid, too much iodine already, pregnancy or breastfeeding, and allergy to halogens (iodine-type substances). [4] It lists possible interactions with lithium, thyroid medicines, blood-pressure medicines, blood thinners and other iodine-containing medicines. [4] If you have any thyroid condition, take thyroid medicine or lithium, are pregnant or breastfeeding, or have kidney disease, speak to a clinician first. See thyroid conditions and iodine or herbal products and seaweed, iodine and the thyroid.

Overview

Bladderwrack is Fucus vesiculosus L., a brown seaweed. GBIF lists it as the accepted name, in the family Fucaceae, order Fucales. [2] The part used is the thallus, the whole body of the seaweed. [3] Native range: not found in the sources we read. It is sold as powder, capsules, tablets, extracts and as an ingredient in slimming and "thyroid" products. This page covers Fucus vesiculosus. MSK points out that bladderwrack is often called "brown kelp" but is not the same seaweed as true kelp. [1] Other seaweeds, kelp and sea moss are covered in seaweed, sea moss and kelp.

Traditional use (not proof of benefit)

The EU assessment describes older European reference books that listed bladderwrack for obesity, goitre (an enlarged thyroid), myxedema (severe underactive thyroid), rheumatism and arthritis, and for obesity linked to an underactive thyroid. [4] On that basis of long-standing use, the EU granted a "traditional use" status only: a traditional herbal medicine used as an add-on to a reduced-calorie diet to help weight loss in overweight adults, after a doctor has ruled out serious conditions. [3] The EU says this rests only on long-standing use, not on proof from trials, and that there is no "well-established" use. [3][4] MSK lists weight loss, skin care, underactive thyroid, tiredness and menstrual problems as the claimed uses, none confirmed in clinical studies. [1]

What research shows

Lab and animal research has looked at antioxidant, anti-cancer and other effects, but the EU says these findings are not relevant to the traditional weight-loss use. [4] MSK likewise notes these effects come from preclinical studies and human data are limited. [1]

Forms and why the form matters

The EU traditional-use status covers the powdered seaweed in solid oral forms such as capsules, with the iodine content in the dried material set by the European Pharmacopoeia at between 0.03% and 0.2%. [3] Supplements outside that scheme may be extracts, mixed products or seaweed blends with unknown iodine and metal content. The EU report notes that kelp supplements on the market can contain very high iodine amounts (up to 57 mg per dose in tablets). [4] A result or a safety limit for one product cannot be assumed for another. How much to use is a decision for a clinician or pharmacist, not this page.

Safety in more detail

• Thyroid: the EU monograph says intake above recommended amounts can change TSH, and adds that other bladderwrack products, iodine products and thyroid medicines should not be taken at the same time. [3] The EU assessment mentions clinical hyperthyroidism reported in people taking kelp products for slimming or as supplements. [4]
• Official iodine limit: the EU monograph states a limit of 400 micrograms of total iodine per day from bladderwrack medicines, and the assessment recommends regular thyroid monitoring (for example every four months) and a limit on how long treatment lasts. [3][4] NHS says long-term high iodine can affect thyroid function and that supplements of 0.5 mg a day or less are unlikely to cause harm. [8]
• Lithium: a 2011 case report describes a 60-year-old man with bipolar disorder taking lithium who developed hyperthyroidism while using a herbal laxative product containing bladderwrack; it resolved after the product was stopped. [6] One case cannot show how common this is. [4]
• Other medicines: MSK says bladderwrack may affect drugs handled by liver enzymes (CYP450) and that an animal study found lower amiodarone availability; clinical relevance is not known. [1] A rat study found repeated bladderwrack extract lowered peak blood levels of the epilepsy medicine lamotrigine, though the authors expected no significant clinical impact. [7] The EU assessment lists blood thinners as a possible interaction but notes other literature considers an effect on vitamin K antagonists unlikely. [4]
• Arsenic and heavy metals: a 54-year-old woman developed hair loss, memory loss, rash, fatigue, nausea and vomiting, and had a high urine arsenic level, after taking daily kelp supplements; the authors also tested nine kelp supplements from health food stores and found eight with arsenic above an FDA food tolerance level. [5] The EU report notes an industry association objected to that case report. [4] A separate case describes low platelets and abnormal red blood cell production in a person taking kelp for 6 weeks, attributed to its arsenic content. [4]
• Kidneys and electrolytes: the EU assessment lists defects in the kidney's collecting ducts and a case of Fanconi syndrome (a kidney tubule disorder) after ingestion of bladderwrack, and advises caution in kidney disease and high blood pressure. [4]
• Bladder: MSK cites a case of haemorrhagic cystitis (bladder inflammation with bleeding) in a woman taking a slimming product with 20 herbs including bladderwrack, so the cause cannot be pinned on bladderwrack alone. [1]
• Pregnancy and breastfeeding: the EU says safety has not been established and use is not recommended. [3]
• Children: the EU does not recommend use under 18. [3] The assessment lists children under five as a contraindication. [4]
• Anaemia: the assessment notes fucoidan (a seaweed sugar) may reduce iron absorption from the gut, so caution is advised in anaemia. [4]
• Swallowing: the EU notes that swelling seaweed products can block the throat if taken without enough water. [4]
• Liver: the sources we read do not describe a liver warning for bladderwrack. We could not read NIH's LiverTox entry for it, so we make no claim either way.
• Quality: the EU says the quality of the seaweed must be guaranteed, especially for heavy metals, and the iodine content should be known. [4] For label checks see heavy metals in natural ingredients.

Official statements

• EMA (EU), HMPC, 6 May 2014: community herbal monograph and assessment report: traditional use only, as an add-on to a reduced-calorie diet for weight loss in overweight adults; iodine limit of 400 micrograms per day from the medicine; not recommended in pregnancy, breastfeeding or under 18; no genotoxicity data, so no entry on the EU list of herbal substances. [3][4]
• MSK, updated 30 June 2022: purported uses not confirmed in clinical studies; warns of iodine, heavy metals and possible drug effects. [1]
• Not found in the sources we read: a current NCCIH fact sheet or US FDA statement specific to bladderwrack.

Related pages

• Seaweed, sea moss and kelp
• Seaweed, iodine and the thyroid
• Kelp and seaweed supplements: iodine content and risks
• Iodine
• Herbs for weight loss: claims checked
• Herbs before and after surgery