Research review
Bladderwrack: what the research does and doesn't show
A map of the bladderwrack research by outcome: blood sugar, cholesterol, joints, hormones, thyroid and contamination.
The research does not support bladderwrack as a treatment for any condition. The human studies mostly used extracts or blends rather than the whole dried plant, and most found little or nothing. The most consistent finding is about iodine and thyroid-stimulating hormone, which is a risk to manage, not a benefit. This page lists each study by outcome.
What the evidence doesn't support
It does not support bladderwrack for blood sugar, cholesterol, weight, joints, hormones, kidneys or skin. It does not support applying an extract result to the whole plant, because the extracts were concentrated and standardised and the plant is not. It does not support crediting bladderwrack for results from products that also contained chromium, vitamins or other seaweeds. And it does not support any dose as effective, because the amounts studied ranged from 300 mg of an extract to 5 g of dried plant a day. For the narrative, see bladderwrack benefits: what the research does and doesn't show; this is the inventory.
What the trials found
We searched PubMed on 2 October 2026 for Fucus vesiculosus and bladderwrack with trial, case-report and observational filters, and read every abstract below. "Extract" means a concentrated product, not powder. Certainty is for the question "did bladderwrack change this outcome in this group".
| Study | Population | Dose and form | Duration | Result | Certainty |
|---|---|---|---|---|---|
| Blood sugar | |||||
| PMID 34549293 (2022) | Healthy, at-risk and type 2 diabetic adults; 18 trials | Many brown seaweed species and extracts | Varied | Pooled fasting and after-meal glucose were lower, but no high-quality trial was found and dose-response is unknown | Low: mixed species and forms |
| PMID 38931192 (2024) | 36 overweight adults with type 2 diabetes on regular treatment (10 in the bladderwrack arm) | 5 g a day dried bladderwrack | 5 weeks | No difference in average blood glucose or other outcomes versus a control seaweed | Low: tiny pilot |
| PMID 35323474 (2022) | 56 overweight or obese adults with prediabetes | 500 mg a day Ascophyllum plus bladderwrack extract | 12 weeks | No effect on blood glucose or body weight; small differences in C-peptide, heart rate and IL-6 | Low: one trial, marginal changes |
| PMID 29495492 (2018) | 38 healthy adults | 500 mg or 2,000 mg bladderwrack extract, single dose | One dose | Neither dose lowered blood glucose or insulin after a meal | Low |
| PMID 30813480 (2019) | 18 healthy adults | Bladderwrack extract before an evening meal; dose not stated | One dose | No effect overall; lower peak glucose in women only, flagged for further study | Very low |
| PMID 30714233 (2019) | 65 adults with dysglycemia | Seaweed polyphenols plus chromium picolinate | 6 months | Glucose measures were lower than with placebo; chromium is in the product, so it cannot be credited to seaweed | Low |
| Cholesterol and weight | |||||
| PMID 34015499 (2021) | 34 overweight adults with raised LDL cholesterol | 2,000 mg a day bladderwrack extract | 12 weeks | HDL cholesterol rose 9.5% versus placebo; nothing else measured changed; the authors say a larger sample is needed | Low |
| PMID 34631894 (2021) | 505 adults with metabolic syndrome components; retrospective, no control group | Seaweed plus chromium picolinate product | 180 days | Average weight fell 7.3 kg and several blood measures changed; the maker supported the analysis | Very low: no control, chromium in product |
| Joints | |||||
| PMID 27307702 (2016) | 122 adults with hip or knee osteoarthritis | 300 mg a day bladderwrack extract, 85% fucoidan | 12 weeks | Symptom reduction was not different from placebo | Low: one null trial of an extract |
| PMID 20376172 (2010) | 12 adults with knee osteoarthritis | 100 or 1,000 mg a day of an extract blend (85% bladderwrack) with B6, zinc and manganese | 12 weeks | Symptom scores fell 18% and 52%; open-label, no placebo group | Very low |
| Hormones and the menstrual cycle | |||||
| PMID 15294021 (2004) | 3 pre-menopausal women with abnormal cycles or related histories | 700 mg, then 1.4 g a day of bladderwrack | Not stated in abstract | Cycles lengthened by 5.5 to 14 days; estradiol fell and progesterone rose in the one woman measured | Very low: three women, no control; a reason for caution |
| Gut and immune markers | |||||
| PMID 32759709 (2020) | 22 team-sport athletes and 11 healthy adults | 1 g a day fucoidan from bladderwrack and Undaria | 7 days | Stool lysozyme rose about 45% in athletes; no placebo group; funded by the supplier | Very low |
| Thyroid and iodine (the best-documented effect) | |||||
| PMID 14583417 (2003) | 36 healthy adults | 2 or 4 kelp capsules a day; species not stated | 4 weeks | Thyroid-stimulating hormone rose at both doses; urinary iodine rose with dose | Low (kelp, not bladderwrack) |
| PMID 25006699 (2014) | 42 women with low dairy and seafood intake | 0.5 g a day Ascophyllum seaweed | 2 weeks | Urinary iodine and TSH rose; two women went above the normal range afterwards | Low (no placebo group) |
| PMID 31517826 (2019) | One 70-year-old woman | Kelp tablets, dose not stated | About 3 months | Overactive thyroid with rapid heartbeat, insomnia, anxiety and weight loss; resolved after stopping | One case |
| Contamination | |||||
| PMID 17450231 (2007) | One 54-year-old woman; nine kelp products tested | Daily kelp supplement containing 8.5 mg/kg arsenic | Symptoms over 2 years | Arsenic toxicity that resolved after stopping; eight of nine products had arsenic above food tolerance levels | One case plus a small product test |
| PMID 33889064 (2021) | 96 commercial seaweed products | Supplements: 5 to 5,600 mcg iodine per daily dose | Not applicable | 54 products would exceed the iodine upper limit in one portion or dose; labels were often inaccurate | Lab measurement |
Where the evidence is, and isn't
The evidence is strongest on one point: seaweed iodine changes thyroid-stimulating hormone, in controlled and uncontrolled studies. It is weakest where marketing is loudest: weight and blood sugar. Among the eight studies that reported glucose, cholesterol or weight, the controlled trials of a single seaweed extract or dried plant were null or marginal (one HDL change), the meta-analysis pooled many species and found no high-quality trial, and the two positive product studies also contained chromium. Whole dried plant at a capsule-sized amount has one tiny uncontrolled study, in three women. See bladderwrack side effects and bladderwrack and medications.
Questions people ask
Can bladderwrack cause weight gain?
No study we found reports weight gain from bladderwrack. Where body weight was measured in controlled trials, it did not change (PMID 35323474). Seaweed iodine can change thyroid hormones, which is why unexplained weight change on a seaweed supplement is a reason to see a doctor, not to adjust the dose yourself.
Can bladderwrack cause diarrhea, constipation or headache?
None of the abstracts we read reports any of the three as a result, so we cannot give a frequency. The documented problems are thyroid changes and, in one case, arsenic contamination in a kelp supplement. If you get new digestive symptoms or headaches after starting a seaweed product, stop and ask a doctor or pharmacist.
Is bladderwrack safe during pregnancy, while breastfeeding, or for kids?
We found no study of bladderwrack in pregnancy, breastfeeding or children, and its iodine content is unpredictable, so we cannot call it safe. This page gives no guidance on any of them. Ask a doctor first.
When to talk to a doctor
Bladderwrack is not a treatment for any condition. Talk to a doctor or pharmacist before taking it if you have a thyroid condition or take thyroid medication, have a hormone-sensitive condition, or are pregnant or breastfeeding. Seek care promptly for a racing heartbeat, tremor or unexplained weight change while taking a seaweed supplement.
Sources
- Effect of brown seaweed on plasma glucose in healthy, at-risk, and type 2 diabetic individuals: systematic review and meta-analysis A review of 18 trials of brown seaweeds (many species and extracts) found a pooled glucose difference but varied results and no high-quality trial, and called for species- and dose-specific research.
- 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 In 36 adults with type 2 diabetes, 5 g a day of dried bladderwrack for 5 weeks did not change average blood glucose.
- Marginal Impact of Brown Seaweed Ascophyllum nodosum and Fucus vesiculosus Extract on Metabolic and Inflammatory Response in Overweight and Obese Prediabetic Subjects In 56 overweight adults with prediabetes, 500 mg a day of a brown-seaweed extract for 12 weeks did not change blood glucose or body weight.
- The Impact of a Single Dose of a Polyphenol-Rich Seaweed Extract on Postprandial Glycaemic Control in Healthy Adults: A Randomised Cross-Over Trial In 38 healthy adults, a single 500 mg or 2,000 mg dose of bladderwrack extract did not lower blood glucose after a meal.
- A 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 In 18 healthy adults, a single evening dose of bladderwrack extract did not lower blood glucose overall; a drop in peak glucose in women only was flagged for further study.
- Ascophyllum nodosum and Fucus vesiculosus on glycemic status and on endothelial damage markers in dysglicemic patients In 65 adults with dysglycemia, a seaweed-plus-chromium combination for 6 months was followed by lower glucose measures than placebo; chromium confounds it.
- Twelve 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 In 34 overweight adults with high LDL, 2,000 mg a day of a bladderwrack extract for 12 weeks raised HDL slightly; nothing else changed.
- Effectiveness of Ascophyllum nodosum and Fucus vesiculosus on Metabolic Syndrome Components: A Real-World, Observational Study A 505-person uncontrolled study of a seaweed-and-chromium product reported an average 7.3 kg weight loss over 6 months; with no control group and chromium in the product, it can't be attributed to bladderwrack.
- Effects of fucoidan from Fucus vesiculosus in reducing symptoms of osteoarthritis: a randomized placebo-controlled trial In 122 adults with hip or knee osteoarthritis, 300 mg a day of a bladderwrack fucoidan extract for 12 weeks did no better than placebo and was well tolerated.
- A combined phase I and II open label study on the effects of a seaweed extract nutrient complex on osteoarthritis In 12 people with knee osteoarthritis, a seaweed extract blend plus vitamins and minerals was followed by lower symptom scores in a dose-dependent way; there was no placebo group.
- The effect of Fucus vesiculosus, an edible brown seaweed, upon menstrual cycle length and hormonal status in three pre-menopausal women: a case report In 3 pre-menopausal women, 700 mg to 1.4 g a day of bladderwrack lengthened menstrual cycles; there was no control group.
- Fucoidan Supplementation Restores Fecal Lysozyme Concentrations in High-Performance Athletes: A Pilot Study In 33 people, 7 days of 1 g a day of a Fucus/Undaria fucoidan extract was followed by higher fecal lysozyme in athletes; no placebo group.
- Effects of kelp supplementation on thyroid function in euthyroid subjects In 36 healthy adults, 4 weeks of kelp capsules raised TSH in both the low-dose and high-dose groups, and most values returned to baseline after stopping.
- Low-level seaweed supplementation improves iodine status in iodine-insufficient women In 42 women, 0.5 g a day of a brown seaweed (Ascophyllum nodosum, not bladderwrack) for 2 weeks raised iodine excretion and TSH, with two women above the normal range.
- Transient Hyperthyroidism following the ingestion of complementary medications containing kelp seaweed: A case-report A case report describes a 70-year-old woman with no prior thyroid disease who developed temporary thyroid overactivity three months after starting a kelp seaweed supplement, which resolved after stopping it.
- Case report: potential arsenic toxicosis secondary to herbal kelp supplement One woman's two-year illness was traced to arsenic in a kelp supplement and resolved after she stopped it; 8 of 9 kelp products tested exceeded the FDA food tolerance level.
- Commercially available kelp and seaweed products - valuable iodine source or risk of excess intake? A lab test of 96 seaweed products found iodine in supplements ranging from 5 to 5,600 mcg per daily dose, and labels were often inadequate or inaccurate.
*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.