Bladderwrack

Also called Fucus vesiculosus, black tang, rockweed, bladder fucus, brown seaweed.

Best grade D across 6 graded outcomes Botanical Last checked Oct 21, 2026

The short answer

From 17 sources

Bladderwrack is a brown seaweed sold for metabolism and thyroid support. In small trials its polyphenol-rich extract did not lower blood sugar, and a 12-week trial found no change in LDL cholesterol. Whole seaweeds can carry large, variable amounts of iodine, and kelp supplements have been linked to hyperthyroidism in case reports.

Every graded outcome on this page

6 rows · line = participants

Grade DBlood glucose and weight loss with a low-calorie dietBlood sugarn 56
Grade DBlood glucose and insulin after breadBlood sugarn 38
Grade DAverage blood glucose on continuous monitoringBlood sugarn 36
Grade DCholesterol, glucose, inflammation, cognition and moodCholesteroln 34
Grade DGlucose and insulin after a carbohydrate mealBlood sugarn 23
Grade DEvening blood glucose after breadBlood sugarn 18
3 more graded outcomes, in the evidence chapter

Key facts

What it is: An edible brown seaweed (Fucus vesiculosus), sold dried, as an extract, or blended with Ascophyllum or sea moss^1,4
Blood sugar: Single 500 to 2,000 mg doses of extract did not lower glucose after meals; 5 g/day of dried seaweed did not change glucose in type 2 diabetes^2,3,4
Pooled seaweed trials: A 2023 meta-analysis of 23 studies of several brown seaweeds found lower post-meal glucose, HbA1c and insulin resistance, but no change in fasting glucose¹⁴
Cholesterol: 2,000 mg/day for 12 weeks raised HDL 9.5% but did not change LDL or other risk markers¹
Iodine: Iodine in seaweeds ranged from 16 to over 8,000 mcg per gram across samples; the adult upper limit is 1,100 mcg a day⁸
Thyroid case reports: Kelp-containing supplements were followed by transient hyperthyroidism in published case reports^9,10
Breastfeeding: Iodine is actively transported into breast milk; infants of mothers eating large amounts of brown seaweed developed hypothyroidism¹²

What it is

Bladderwrack (Fucus vesiculosus) is a brown seaweed from North Atlantic coasts, sold dried or as an extract and often in sea moss blends.

How it works

Proposed

Bladderwrack contains polyphenols (phlorotannins), the fiber fucoidan and iodine. Brown seaweed blends inhibit the starch-digesting enzymes alpha-amylase and alpha-glucosidase, the proposed basis for blood-sugar claims, but extract trials found little effect on glucose. Its iodine, which varies widely between seaweed samples, is the basis for thyroid claims and for the safety concern.^2,5,8

Forms, side by side

3 forms named in our sources. The rest show what our data holds, and say plainly where it holds nothing yet.

Bladderwrack forms compared

3 forms

Bladderwrack forms compared: 3 forms by 3 attributes. Empty cells are labelled No data yet, or Not graded in the evidence row.
3 forms	Polyphenol-rich bladderwrack extract	Dried whole bladderwrack (powder or capsules)	Ascophyllum nodosum and bladderwrack blend
Absorption in people	No head-to-head human data.	No head-to-head human data.	No head-to-head human data.
GI tolerance	The abstracts of the extract trials do not report adverse effects.^1,2,3	Whole seaweeds can carry large and variable amounts of iodine; kelp supplements have been linked to transient hyperthyroidism in case reports.^8,9,10	The abstracts do not report adverse effects.^5,6
Studied for	Cholesterol and blood sugar	Blood sugar in type 2 diabetes (pilot)	Insulin and blood sugar after meals; prediabetes

Polyphenol-rich bladderwrack extract
Absorption in people
No head-to-head human data.
GI tolerance
The abstracts of the extract trials do not report adverse effects.^1,2,3
Studied for
Cholesterol and blood sugar
Dried whole bladderwrack (powder or capsules)
Absorption in people
No head-to-head human data.
GI tolerance
Whole seaweeds can carry large and variable amounts of iodine; kelp supplements have been linked to transient hyperthyroidism in case reports.^8,9,10
Studied for
Blood sugar in type 2 diabetes (pilot)
Ascophyllum nodosum and bladderwrack blend
Absorption in people
No head-to-head human data.
GI tolerance
The abstracts do not report adverse effects.^5,6
Studied for
Insulin and blood sugar after meals; prediabetes

What the human research says

Human trials only, one row per outcome. The form line shows what each trial actually tested. Grades follow our evidence-grading policy.

Trial map

6 graded outcomes, largest study first

6 graded outcomes: 2 mixed, 4 no effect. The largest, a 2022 randomized trial of 56 people on blood glucose and weight loss with a low-calorie diet, found no effect.

By sizeTable

1Blood glucose and weight loss with a low-calorie diet

No effect · Grade D · RCT, 56 people, 2022

Overweight or obese adults with prediabetes and insulin resistance (Canada)

Dose, result and source

Every graded outcome on this page, largest study first
Outcome	Found	People	Study	Who was studied
Blood glucose and weight loss with a low-calorie diet	No effect	56	Grade D · RCT, 2022	Overweight or obese adults with prediabetes and insulin resistance (Canada)
Blood glucose and insulin after bread	No effect	38	Grade D · Crossover RCT, 2018	Healthy adults aged 19 to 56 (Australia)
Average blood glucose on continuous monitoring	No effect	36	Grade D · Pilot RCT, 2024	Overweight adults with type 2 diabetes (Netherlands)
Cholesterol, glucose, inflammation, cognition and mood	Mixed	34	Grade D · RCT, 2021	Overweight or obese adults with raised LDL cholesterol (Australia)
Glucose and insulin after a carbohydrate meal	Mixed	23	Grade D · Crossover RCT, 2011	Healthy men and women aged 19 to 59 (Canada)
Evening blood glucose after bread	No effect	18	Grade D · Crossover RCT, 2019	Healthy adults (Australia)

Every graded row

6 rows

Form tested

Showing 6 of 6 outcomes. Human trials only.

Grade DBlood glucose and weight loss with a low-calorie dietBlood sugarNo effectRCT
n 56 · 2022

No effect No effect on blood glucose; only small changes in C-peptide during a glucose test were reported, and the authors described the impact as marginal.

Design: RCT, n = 56, 2022
Population: Overweight or obese adults with prediabetes and insulin resistance (Canada)
Form: Ascophyllum nodosum and Fucus vesiculosus extract
Dose: 500 mg/day with diet advice (vs placebo)
Duration: 12 weeks
Source: Reference 6 · PubMed 35323474 (opens PubMed)

Grade DBlood glucose and insulin after breadBlood sugarNo effectCrossover RCT
n 38 · 2018

No effect Neither dose lowered glucose or insulin after the meal compared with placebo.

Design: Crossover RCT, n = 38, 2018
Population: Healthy adults aged 19 to 56 (Australia)
Form: Polyphenol-rich brown seaweed (Fucus vesiculosus) extract
Dose: 500 mg or 2,000 mg single dose before 50 g carbohydrate
Duration: Single doses
Source: Reference 2 · PubMed 29495492 (opens PubMed)

Grade DAverage blood glucose on continuous monitoringBlood sugarNo effectPilot RCT
n 36 · 2024

No effect Weekly average glucose did not change significantly in the Fucus group (n = 10) and did not differ between groups.

Design: Pilot RCT, n = 36, 2024
Population: Overweight adults with type 2 diabetes (Netherlands)
Form: Dried whole Fucus vesiculosus (vs Sargassum fusiforme or a Porphyra control)
Dose: 5 g/day of dried seaweed
Duration: Not stated in the abstract
Source: Reference 4 · PubMed 38931192 (opens PubMed)

Grade DCholesterol, glucose, inflammation, cognition and moodCholesterolMixedRCT
n 34 · 2021

Mixed HDL cholesterol rose 9.5% versus placebo, but LDL, total cholesterol, triglycerides, glucose, insulin, inflammatory markers, cognition and mood did not change.

Design: RCT, n = 34, 2021
Population: Overweight or obese adults with raised LDL cholesterol (Australia)
Form: Polyphenol-rich Fucus vesiculosus extract
Dose: 2,000 mg/day vs placebo
Duration: 12 weeks
Source: Reference 1 · PubMed 34015499 (opens PubMed)

Grade DGlucose and insulin after a carbohydrate mealBlood sugarMixedCrossover RCT
n 23 · 2011

Mixed The insulin response was 12.1% lower and an insulin-sensitivity index 7.9% higher than placebo, but the glucose response did not change.

Design: Crossover RCT, n = 23, 2011
Population: Healthy men and women aged 19 to 59 (Canada)
Form: Ascophyllum nodosum and Fucus vesiculosus blend (InSea2)
Dose: 500 mg single dose, 30 minutes before 50 g carbohydrate
Duration: Single doses, 1-week washout
Source: Reference 5 · PubMed 22087795 (opens PubMed)

Grade DEvening blood glucose after breadBlood sugarNo effectCrossover RCT
n 18 · 2019

No effect No effect on evening glucose response in the group as a whole; peak glucose was lower in women only, a subgroup finding.

Design: Crossover RCT, n = 18, 2019
Population: Healthy adults (Australia)
Form: Polyphenol-rich Fucus vesiculosus extract
Dose: Single dose before 50 g carbohydrate in the evening (vs two placebos)
Duration: Single doses
Source: Reference 3 · PubMed 30813480 (opens PubMed)

What the numbers add up to

Each point is worked out from the data and the sources numbered on this page, with the arithmetic shown under it. It adds no new data and is not medical advice.

The metabolism claim did not hold up

Three trials of the same polyphenol-rich extract found no lowering of blood sugar after meals and no change in LDL cholesterol over 12 weeks.^1,2,3

Thyroid risk comes from iodine

Products sold for thyroid support rely on iodine, but seaweed iodine content is unpredictable, and excess iodine has been linked to hyperthyroidism in case reports.^8,9,10

Studied doses

Doses used in research, shown against the official limits. They describe the studies, not a recommendation for you.

Studied doses in research, not advice. These are amounts used in research, not personal advice. Your clinician or pharmacist can say what fits you.

Doses studied in human trials

Doses studied in human trials ranged from 500 to 5,000 mg/day: single 500 to 2,000 mg doses of extract before a meal, 2,000 mg/day of extract for 12 weeks, 500 mg/day of a blend with Ascophyllum for 12 weeks, and 5 g/day of dried whole seaweed in a type 2 diabetes pilot.^1,2,4,5,6

Interactions and safety

Published interactions only: each row cites a source. No row means none was found in our sources, not that a combination is safe.

What Bladderwrack meets

3 cited rows

Medicines 2

Bladderwrack2 medicines · 1 supplement

Supplements and foods 1

Reference summary

Interactions

3 agents

Published interactions, grouped by kind
Agent	What the source says	Evidence
Medicines2
Thyroid medicinese.g. levothyroxine, methimazole	Excess iodine can cause hypothyroidism or hyperthyroidism, especially in people with underlying thyroid disease, and endocrinologists report patients with iodine-induced thyroid dysfunction from kelp-based supplements. No study has tested bladderwrack with these medicines; the concern is inferred from its iodine content.^16,17	Review
Amiodarone and iodinated contraste.g. Cordarone; contrast dyes for CT scans	Endocrinology reviews list amiodarone and iodinated contrast, alongside seaweed, as sources of iodine excess that can cause thyroid dysfunction. This is an inference: no study has tested bladderwrack with either, but seaweed iodine would add to the same load.^15,16	Review
Other supplements1
Iodine supplements and iodine-containing multivitaminse.g. potassium iodide, kelp tablets	Seaweed iodine adds to iodine from supplements; seaweed samples alone ranged from 16 to over 8,000 mcg of iodine per gram, against an adult upper limit of 1,100 mcg a day.^8,15	Review

Check with what you take

Kept on this device only

Add what you take. We list every interaction our evidence profiles cite between Bladderwrack and your picks, and between your picks.

Add a supplement or medicine

Most cited:

Add a supplement or medicine you take to check it against Bladderwrack.

Starting from one medicine? The interaction checker rates how serious each pair is, on a curated list of 19 entries for 11 medicines. This cabinet has no severity rating: it lists every interaction row our supplement pages cite.

Who should be careful

5 groups

Thyroid conditions: People with underlying thyroid disease are most susceptible to iodine-induced thyroid dysfunction, and seaweed is a recognised source of iodine excess. Seaweed iodine content varies widely and can exceed the adult upper limit of 1,100 mcg a day, and kelp supplements have been followed by transient hyperthyroidism in case reports.^16,15,8,9,10
Heavy metals: Seaweeds absorb metals from seawater, and LactMed notes bladderwrack can contain arsenic, cadmium and lead. A woman's hair loss, rash and memory problems resolved after she stopped a kelp supplement that contained 8.5 ppm arsenic, and 8 of 9 store-bought kelp supplements tested above FDA arsenic tolerance levels for certain foods.^12,11
Pregnancy and breastfeeding: LactMed's seaweed entry, which covers bladderwrack, notes that iodine is actively transported into breast milk and heavy metals pass into milk; some sources advise against seaweed while breastfeeding. Infants of mothers who ate large amounts of brown-seaweed soup developed high TSH or hypothyroidism. No cited trial enrolled pregnant women, and seaweed iodine content is unpredictable.^12,1,4,8
Liver: LiverTox's kelp entry, which covers Laminaria kelps and lists seaweed and brown algae as other names, rates kelp likelihood score E (an unlikely cause of liver injury). LiverTox has no separate bladderwrack entry.¹³
Hormone-sensitive conditions: In a case report of three premenopausal women, bladderwrack lengthened menstrual cycles by 5.5 to 14 days and lowered estradiol in one woman.⁷

Reading the label, and the cost per dose

Compare any product you already have by the amount it delivers, not by capsule count or front-of-pack weight.

How to read the label

3 checks

Whole seaweed vs extract

Most trials used a polyphenol-rich extract. Whole dried bladderwrack, common in sea moss blends, was tested only in one 5 g/day pilot and can carry far more iodine.^1,4,8

Iodine is rarely on the label

Iodine in seaweed varies with species, harvest and processing, so a label without a tested iodine amount gives no reliable figure.⁸

Look for heavy-metal testing

Kelp supplements tested in a 2007 report often carried arsenic and none mentioned possible heavy-metal contamination on the label, so a certificate of analysis for arsenic and other metals matters.¹¹

Have the bottle? Photograph its Supplement Facts panel and see each line, bladderwrack included, beside the doses and forms the trials used in Label X-Ray.

Cost per studied dose, without the sales pitch

Your numbers, no product data

Price paid ($)Servings per containerBladderwrack per serving (mg)

Compare against

Enter the price, servings and the amount per serving from your label.

Frequently asked questions

Short answers to common questions; the numbers link to the sources below.

Common questions

7 answers

Does bladderwrack help with weight or metabolism?

No trial in our sources showed weight loss. Extract trials did not lower blood sugar after meals, and a 500 mg/day blend added to a diet had a marginal effect in prediabetes.^2,3,6

Is bladderwrack good for the thyroid?

Its iodine content is the basis for that claim, but seaweed iodine varies widely, and excess iodine can cause both overactive and underactive thyroid, especially in people with thyroid disease. Kelp supplements have been followed by transient hyperthyroidism in case reports.^8,16,9,10

Does bladderwrack lower cholesterol?

In a 12-week trial in 34 adults, 2,000 mg/day of extract did not change LDL or total cholesterol; HDL rose 9.5% versus placebo.¹

Can bladderwrack affect hormones?

A case report in three premenopausal women found longer menstrual cycles and, in one woman, lower estradiol. That is too little to draw firm conclusions.⁷

Is it safe to take bladderwrack every day?

Long-term safety is not established: the longest trial in our sources lasted 12 weeks at 2,000 mg/day of extract. The main risks of daily use are excess iodine and heavy metals such as arsenic, both of which vary by product.^1,8,11,12

Why is bladderwrack in sea moss blends?

It is a brown seaweed often combined with sea moss. No trial in our sources tested those blends.^4,8

How much iodine is in bladderwrack?

It is unpredictable. A survey of edible seaweeds found 16 to over 8,000 mcg of iodine per gram across samples, and labels rarely give a tested amount. The adult upper limit is 1,100 mcg a day, so a few grams of a high-iodine seaweed could exceed it.⁸

Sources

17 sources: trial abstracts, NIH fact sheets and reference databases. Numbers in the text link here.

Sources cited on this page

17 sources

1Twelve weeks' treatment with a polyphenol-rich seaweed extract increased HDL cholesterol with no change in other biomarkers of chronic disease risk in overweight adults: A placebo-controlled randomized trial.RCT J Nutr Biochem, 2021 · PMID 34015499
2The Impact of a Single Dose of a Polyphenol-Rich Seaweed Extract on Postprandial Glycaemic Control in Healthy Adults: A Randomised Cross-Over Trial.Crossover RCT Nutrients, 2018 · PMID 29495492
3A Single-Dose of a Polyphenol-Rich Fucus Vesiculosus Extract is Insufficient to Blunt the Elevated Postprandial Blood Glucose Responses Exhibited by Healthy Adults in the Evening: A Randomised Crossover Trial.Crossover RCT Antioxidants (Basel), 2019 · PMID 30813480
4The 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.Pilot RCT Nutrients, 2024 · PMID 38931192
5A randomised crossover placebo-controlled trial investigating the effect of brown seaweed (Ascophyllum nodosum and Fucus vesiculosus) on postchallenge plasma glucose and insulin levels in men and women.Crossover RCT Appl Physiol Nutr Metab, 2011 · PMID 22087795
6Marginal Impact of Brown Seaweed Ascophyllum nodosum and Fucus vesiculosus Extract on Metabolic and Inflammatory Response in Overweight and Obese Prediabetic Subjects.RCT Mar Drugs, 2022 · PMID 35323474
7The effect of Fucus vesiculosus, an edible brown seaweed, upon menstrual cycle length and hormonal status in three pre-menopausal women: a case report.Observational BMC Complement Altern Med, 2004 · PMID 15294021
8Variability of iodine content in common commercially available edible seaweeds.Observational Thyroid, 2004 · PMID 15588380
9Transient Hyperthyroidism following the ingestion of complementary medications containing kelp seaweed: A case-report.Observational Medicine (Baltimore), 2019 · PMID 31517826
10Transient hyperthyroidism in a patient taking dietary supplements containing kelp.Observational J Am Board Fam Pract, 1998 · PMID 9876004
11Case report: potential arsenic toxicosis secondary to herbal kelp supplement.Observational Environ Health Perspect, 2007 · PMID 17450231
12SeaweedDatabase LactMed (NCBI Bookshelf), 2024 · ncbi.nlm.nih.gov
13KelpDatabase LiverTox (NCBI Bookshelf), 2024 · ncbi.nlm.nih.gov
14Brown Seaweed Consumption as a Promising Strategy for Blood Glucose Management: A Comprehensive Meta-Analysis.Meta-analysis Nutrients, 2023 · PMID 38068845
15Iodine and Hyperthyroidism: A Double-Edged Sword.Review Endocr Pract, 2025 · PMID 39510448
16Iodine-induced thyroid dysfunction.Review Curr Opin Endocrinol Diabetes Obes, 2012 · PMID 22820214
17What Endocrinologists Should Know About Iodine: Population Deficiency, Individual Excess and Misinformation in the United Kingdom.Review Clin Endocrinol (Oxf), 2026 · PMID 42144726

What changed

Every dated change to this page: what changed, who made it and why.

Change log

4 entries

Dated changes to this page
Date	Change	Who and why
Oct 21, 2026key facts, interactions, who should avoid, FAQ, questions, related	Interactions were empty: added thyroid medicines, amiodarone and iodinated contrast, and iodine supplements, from endocrinology reviews of iodine-induced thyroid dysfunction (2012, 2025, 2026) and the seaweed iodine survey. Added LactMed (seaweed entry covering bladderwrack: iodine and heavy metals pass into milk; infant hypothyroidism after heavy maternal seaweed intake) and LiverTox (kelp, likelihood score E; bladderwrack not assessed separately). Cited a 2023 meta-analysis of 23 brown-seaweed studies in key facts only (no participant total; several species). Added FAQ on iodine content and daily use, page questions and a goal link. Evidence search for 2023–2026 Fucus vesiculosus trials found nothing newer than the 2024 pilot already in the table.	Writer: Kymata Health EditorialTrigger: A–Z audit against docs/supplements/AUDIT-STANDARD.md
Oct 17, 2026safety	Independent review: added a heavy-metal (arsenic) safety note and label tip from a PubMed-verified kelp supplement report; corrected a note on the 2018 glucose trial, whose abstract does name Fucus vesiculosus.	Writer: Kymata Health EditorialTrigger: review
Oct 17, 2026all	First draft. Every PMID checked against NCBI E-utilities esummary (title and year taken from the record) and abstracts read for every cited row.	Writer: Kymata Health EditorialTrigger: new page
Oct 17, 2026all	Adversarial review pass: every evidence row's numbers machine-checked against its PubMed abstract, retraction and erratum flags checked for all PMIDs, unverified country and affiliation details removed, and claims not supported by the cited abstracts reworded or dropped.	Writer: Kymata Health EditorialTrigger: review

Supplement & Nutrient Encyclopedia. Educational information only; it is not medical advice and does not replace a clinician. Dietary supplements are not intended to diagnose, treat, cure or prevent any disease. Statements about supplements have not been evaluated by the FDA.

Bladderwrack

Medicines 2

Supplements and foods 1

Amiodarone and iodinated contrast

Thyroid medicines

IodineIodine page