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Scientific Name(s): Ascophyllum, Chlorophceae (green algae), Chondrus crispus (Irish moss; red algae), Chondrus, Ecklonia, Fucus, Gelidium, Gracilaria, Hijikia fusiformis (hijiki; brown algae), Palmaria palmate (dulse; red algae), Phaeophyceae (red algae), Porphyra (nori, red algae), Pterocladia, Rhodophyceae (red algae), Saccharina (kombu; brown algae), Sargassum horneri (brown algae), Ulva fasciata (limu palahalaha; green algae), Ulva lactuca (sea lettuce), Undaria pinnatifida (wakame; brown algae)
Common Name(s): Algae, Brown algae, Brown seaweed, Dulse/dillisk, Egg wrack, Green algae, Hijiki, Irish moss/carrageenin, Kelp, Kombu/konbu, Makonbu, Nori, Red algae, Red seaweed, Sea grass, Sea lettuce, Sea spaghetti, Seaweed, Wakame

Medically reviewed by Last updated on Sep 30, 2021.

Clinical Overview


Note: This monograph focuses on all seaweed species, except Laminaria. For more information regarding clinical uses, interactions, adverse reactions, and toxicology associated with Laminaria, see the Laminaria monograph.

Robust clinical data are generally lacking to support definitive therapeutic recommendations for seaweeds. Immune support for seasonal allergic rhinitis and seasonal influenza vaccination have been reported in single moderately sized trials, as well as depressive symptoms related to anhedonia. Results for metabolic syndrome and related cardiovascular outcomes are equivocal.


Doses vary widely depending on the seaweed species and/or extract used. Beneficial dosing in clinical trials have ranged from Ulva lactuca 130 mg/day for 84 days to Ecklonia cava 1,500 mg/day for 12 weeks.


Contraindications have not been identified.


Information regarding safety and efficacy in pregnancy and lactation is lacking.


Patients taking warfarin and consuming a large quantity of food containing seaweed may experience a change in international normalized ratio (INR) because of seaweed's high vitamin K content.

Adverse Reactions

Contact dermatitis, goiter, and, occasionally, GI effects may occur. Transient hypothyroidism due to excess iodine from seaweed has been reported, including neonatally, induced by excessive maternal consumption. False-positive elevation in beta-D-glucan levels has occurred.


Excessive intake of dried seaweed may result in increased serum thyroid-stimulating hormone (TSH). There have been case reports of carotenodermia (yellowing of the skin) with excessive seaweed consumption.


Seaweeds are marine algae, saltwater-dwelling, simple plants, including red (rhodophyta), brown (phaeophyta), and green (chlorophyta) algae. Most algae have root-like structures called holdfasts that anchor the plant to rocks and other substrates.MacArtain 2007 While blue-green species of algae or aquatic plants, such as spirulina (cyanophyta), are actually photosynthetic bacteria. For information on Spirulina, see the individual monograph.

The Japanese term and common name for seaweed, kombu/konbu, should not be confused with kombucha. See Kombucha monograph.


For centuries, seaweed has inspired botanical, industrial, and pharmaceutical interest. Because of the high nutrient content, seaweed has been used as food throughout Asia.

Traditional Chinese medicine used hot water extracts of several types of seaweed in the treatment of cancer. Additionally, the Japanese and Chinese cultures used seaweed to treat goiter and other glandular problems as long ago as 300 BC.

The Romans used seaweed in the treatment of wounds, burns, and rashes.Hocman 1989 The Celts noted that seaweed contracted as it dried and then expanded with moisture. In Scotland during the 18th century, physicians used dried seaweed stem to successfully drain abdominal wall abscesses. They also inserted seaweed into the cervix in an attempt to treat dysmenorrhea. Many reports outline the use of seaweed to induce abortion. Seaweed was employed intravaginally for vaginal atresia and was used urethrally and rectally for strictures.Hocman 1989, Abdussalam 1990, Johnson 1990


Analyses of various seaweeds has identified high carbohydrate levels, as well as minerals, vitamins, and trace elements (eg, calcium, magnesium, iodine, selenium), making them an important nutritional resource, especially when compared with terrestrial sources of trace elements and minerals. The major polysaccharides of seaweed are either structural (contributing to the fiber content) or storage components, including alginates, agars, and carrageenans. The metal salts of alginic acid readily dissolve in cold water to yield viscous solutions. Agars are polysaccharides derived from red seaweed and consist of alternating D- and L-galactopyranose units. Carrageenan, in contrast to agar, is built up from D-galactopyranose units only.MacArtain 2007, Teas 2009 Beta-glucan has also been extracted from brown seaweed.Bobadilla 2013

Uses and Pharmacology

The uses described in the following sections focus on all seaweed species except Laminaria. See Laminaria monograph for further information regarding its potential uses.

Due to the widespread consumption of seaweed as a food source, data from animal studies are largely irrelevant. However, despite the many purported clinical applications for seaweed preparations, only a few, small clinical trials have been conducted by a limited number of researchers.


A double-blind, vehicle-controlled study of a seaweed-derived complex in 60 males younger than 25 years supports safety and efficacy of a commercially available “anti-spot” cosmetic for reducing papules, pustules, and comedomes related to mild acne. Subjects applied the base cream, with or without the active ingredient (0.1% zinc pyrrolidone carboxylic acid and kelp-derived oligosaccharide), twice daily, and although both groups experienced a significant reduction in clinical signs, the benefit in the active treatment group was greater than the control.(Capitanio 2012)


A clinical study evaluated the effect of undaria seaweed among patients with active and latent herpes simplex infections. Lesion healing rates were observed to be increased in those with active infections and, in vitro, undaria inhibited the virus.(Cooper 2002) A review of in vitro studies has been published, describing the effect of sulfated seaweed polysaccharides on herpes simplex and other encapsulated viruses. The polysaccharide may form a complex, blocking the interaction of the virus and cell membranes.(Damonte 2004, Feldman 1999)

A nasal spray containing 0.12% iota-carrageenan from red seaweed administered for 7 days to children (mean age, 5 years) with acute rhinitis significantly reduced viral load, time to clearance of disease, and lowered the incidence of secondary infections with other respiratory viruses. The primary end point (total symptom score) for this randomized, double-blind, placebo-controlled study, however, was not found to be significantly different between treatment and placebo groups.(Fazekas 2012)


The polysaccharides alginate and chitin derived from algae are used in dressings and as topical agents for wound management as well as to treat esophageal reflux. They are being evaluated further for use in drug delivery and tissue regeneration systems.(Nelson 207, d'Ayala 2008)


Animal data

Numerous components of seaweeds have demonstrated varying degrees of antioxidant capability and carcinogen inhibition in vitro and in vivo, such as sulfated polysaccharides (including fucoidan), phenolic compounds (including phlorotannins and bromophenols), and the carotenoid fucoxanthin. Wakame, mekabu, and kelp were found to inhibit dimethylbenz(a)anthracene (DMBA)-induced breast, liver, and skin cancer in rats. Diets supplemented with powdered seaweeds, including Laminaria and Porphyra spp, significantly reduced the incidence of mammary and intestinal tumors in rats.(Park 2013)

Clinical data

Evidence for a preventive effect of seaweed on breast cancer is derived largely from epidemiological studies. Japanese case-control studies found seaweed to be a protective factor against colon cancer (n=100), breast cancer (n=362), and esophageal cancer.(Park 20113) Data comparing women in Japan with the United States suggest the incidence of breast cancer in both pre- and postmenopausal women is lower in Japan and has been attributed in part to higher consumption of seaweed.(Aceves 2005, Teas 2009) More specifically, the importance of co-factor foods or food interactions was described in a double-blind, randomized, placebo-controlled crossover trial in 30 healthy postmenopausal American women when the addition of 5 g/day seaweed powder (Alaria esculenta) to a diet containing 100 g/day soy powder, halved the increase in insulin-like growth factor 1 (IGF-1) caused by the soy. High levels of IGF-1 have been associated with increased risk of breast cancer in postmenopausal women.(Teas 2011)

Effects on serum and urinary estrogen levels have been demonstrated experimentally in small clinical trials, with a dose-dependent relationship demonstrated with seaweed supplementation. This may possibly be due to modulation of colonic bacteria, resulting in increased urinary excretion of 2-hydroxyestrogen.(Teas 2009, Skibola 2004) Clinical trials are lacking.

Cardiovascular/Metabolic syndrome

Increased life expectancy is associated with the Japanese Okinawa diet consisting of low salt intake and consumption of soy beans, fish, and seaweed.(Yamori 2001) Decreases in serum cholesterol and blood pressure are associated with daily consumption of seaweed either as powder or fiber.(Teas 2009) Inhibition of adipocyte differentiation has been reported for the edible Gelidium elegans.(Kim 2019)

In a double-blind, crossover study, 62 middle-aged patients with mild hypertension were given a potassium-releasing seaweed preparation. Mean blood pressure was reduced from 112 to 101 mm Hg after ingestion of 12 g/day seaweed.(Krotkiewski 1991)

In a small study among men and women with metabolic syndrome, decreases in waist circumference (in women only) and blood pressure were demonstrated with daily consumption of 4 to 6 g dried undaria seaweed over 2 months.(Teas 2009)

In a crossover study, a seaweed-derived sodium alginate preparation decreased the mean energy intake, leading researchers to consider the contribution of energy intake to obesity.(Paxman 2008) Because dietary fiber consumption has been associated with decreased energy intake and weight reduction, a four-way double-blind, placebo-controlled, crossover trial (n = 19) evaluated the effects of a preload low- and high-volume 3% alginate extract beverage (from Laminaria hyperborea and Lessonia trabeculata) on satiety, energy intake, and gastric emptying rate in healthy Danish subjects. Satiety and fullness were significantly increased, while hunger and prospective food intake were significantly decreased with the high-volume alginate treatment versus high-volume control. A significant reduction in energy intake was documented for the low-volume, but not the high-volume, treatment. High-volume alginate preload also resulted in a significant (40%) reduction in glucose response, whereas neither treatment resulted in changes to blood pressure or heart rate.(Georg Jensen 2012) In another small trial (n = 10), a reduction in energy intake of 16.4% versus control was significantly lower following consumption of Ascophyllum nodosum 4% enriched wholemeal bread in overweight males.(Hall 2012)

A small (n = 23) randomized, double-blind, crossover study reported positive changes in post-load plasma insulin concentrations (7.9% increase), suggesting improvement in insulin sensitivity subsequent to an acute 500 g dose of brown seaweed capsules (A. nodosum and Fucus vesiculosus; at least 10% polyphenols with alginates removed) prior to ingestion of a 50 g carbohydrate test meal. No significant effect was noted on glucose response.(Paradis 2011) Although, within group decreases in insulin and C-peptide were significant after a 12-week regimen of a dieckol-rich extract from Ecklonia cava (1,500 mg/day), the only significant between-group difference observed was in postprandial glucose (P<0.05) when compared to placebo in a double-blind, randomized, controlled trial in 80 prediabetic patients.(Lee 2015)

No significant overall effect was found on lipid parameters, C-reactive protein, inflammatory markers, or habitual food consumption patterns in 80 overweight or obese participants after 8 weeks of consuming brown algae (A. nodosum) polyphenol-rich extract 400 mg/day and 8 weeks of placebo in a double-blind, randomized, crossover trial. However, a significant reduction in basal DNA damage was observed with brown algae extract in obese patients (body mass index [BMI] greater than 30; P=0.044), and overall in men (P=0.009). Total antioxidant capacity was also significantly reduced with the extract, but only in women (P=0.018). Urinary metabolomic studies of the seaweed polyphenol metabolites revealed 3 distinct groupings of low excreters (25%; less than 0.5 mmol), medium excreters (55%; 0.5 to 2 mmol), and high excreters (20%; more than 2 mmol).(Baldrick 2018) Similarly, no significant effects were observed on lipid, glucose, or insulin parameters in a double-blind, randomized, placebo-controlled trial in 78 overweight or obese Korean adults who consumed Gelidium elegans extract 1,000 mg/day or placebo for 12 weeks. However, changes from baseline in several body composition parameters were significantly improved within the extract group compared to changes experienced within the placebo group. These included improvements in mean body weight (−1.17 kg vs +0.69 kg; P<0.0001), BMI (−0.43 vs +0.26; P<0.0001), total body mass (−1.21 kg vs +0.46 kg; P<0.0001), total fat mass (−0.48 kg vs +1.08 kg; P<0.0001), visceral fat (−7.36 cm2 vs +1.45 cm2; P=0.034), subcutaneous fat (−5.67 cm2 vs +11.63 cm2; P=0.0004), and total abdominal fat (−13.03 cm2 vs +13.08 cm2; P<0.0001) for the extract and placebo, respectively, at 12 weeks. Between-group differences in absolute measures were not significantly different.(Kim 2019)

CNS effects

Animal data

Kainate, an excitatory amino acid, has been extracted from seaweed. Researchers studying the effect of epilepsy show that animal models suggest this chemical generates seizures in the hippocampus.(Ben-Ari 2000)

Clinical data

Based on dose-dependent significant benefit in a depression animal model, the effects of U. lactuca (edible algal extract) on depression in patients with anhedonia was explored in a double-blind, randomized, placebo-controlled trial (N=90). The mean age of participants was 40.2 years, 65.1% were women, and the mean BMI was 25.2 with 38.4% overweight and 11.6% obese. Anhedonia was experienced on average 20 weeks a year, and more than 90% experienced severe episodes. Anticipatory (91.8%) and consummatory (70.6%) anhedonia were most common with the majority of events triggered by problems related to work (55.8%). Significant improvements in the psychological state were observed in both the treatment and placebo groups; however, most outcomes plateaued in the placebo group at day 28, whereas improvement continued in the algae group. On day 84, administration of U. lactuca resulted in statistically significant improvements in depressive symptoms (P=0.0389), sleep disturbance (P=0.0219), psychomotor changes (P=0.002), feelings of improvement (P=0.0114), and clinician-assessed improvements (90.9% vs 70.7%, respectively; P=0.0092) compared to placebo. Adverse events were comparable between groups, and no serious event was reported.(Allaert 2018)

GI effects

The American College of Gastroenterology (ACG) clinical guideline for the management of irritable bowel syndrome (IBS) (2021) suggests that soluble, but not insoluble, fiber be used to treat global IBS symptoms (Strong; moderate).(Lacy 2021)


It has been noted that seaweed extracts exert a stimulatory effect on B lymphocytes and macrophages, which may be used clinically for the modulation of immune responses.(Liu 1997, Shan 1999) In a randomized, placebo-controlled, double-blind study (n = 70) in elderly Japanese people living in a nursing home, 300 mg/day of mekabu fucoidan (sulfated polysaccharide seaweed extract) given 4 weeks before the 2009-2010 seasonal influenza vaccine demonstrated an immunostimulatory effect. The seaweed extract group had significantly higher B strain antibody titers at 5 weeks that were maintained over 20 weeks. Additionally, natural killer cell activity in the seaweed group was not significantly different at week 9 or 24 compared with baseline; however, it was lower at week 24 versus week 9 in the placebo group.(Negishi 2013)

Supplementation of chlorophyll c2 (0.7 mg/day) extracted from S. horneri significantly reduced the use of rescue medication in patients with seasonal allergic rhinitis after 8 weeks compared to placebo (P=0.045) in a double-blind, randomized, controlled trial (N=66). However, no significant difference was seen at the 4-week or 12-week time point. Subgroup analysis revealed better responses for those taking the extract prior to pollen dispersal as well as in patients with class 3 or higher IgE levels to cedar pollen. No significant difference was observed between groups in quality-of-life scores at any of the 3 time points.(Fujiwara 2016)


As a source of calcium, magnesium, selenium, and other minerals, seaweed extracts have been evaluated in osteoarthritis. Methodological problems have limited the validity of the studies, but there may be potential value in the supplementation.(Frestedt 2009, Frestedt 2008)


Robust clinical data supporting doses of seaweed that are more effective than placebo are limited.

Depressive symptoms related to anhedonia

U. lactuca (edible algal extract) administered as a single daily dose given with the evening meal for 84 days according to patient weight; 3 capsules (130 mg/day) for patients 50 to 75 kg, 4 capsules for those 70 to 90 kg, and 5 capsules for those 90 to 110 kg.Allaert 2018


300 mg/day of mekabu fucoidan (sulfated polysaccharide seaweed extract) given 4 weeks before the seasonal influenza vaccine.Negishi 2013

Postprandial glucose in prediabetics

E. cava (1,500 mg/day) for 12 weeks.Lee 2015

Seasonal allergic rhinitis

Chlorophyll c2 (0.7 mg/day) extracted from S. horneri for 12 weeks prior to and during the pollen season.Fujiwara 2016

Pregnancy / Lactation

Information regarding safety and efficacy in pregnancy and lactation is lacking.

Effects on serum and urinary estrogen levels have been demonstrated experimentally in small clinical trials, with a dose-dependent relationship demonstrated with seaweed supplementation.Teas 2009, Skibola 2004 An increase in menstrual cycle length has also been demonstrated with the brown kelp, Fucus vesiculosus.Skibola 2004


Vitamin K antagonist: A single case report describes a change in INR in a patient taking warfarin who consumed a large quantity of sushi that contained seaweed. This was thought to be caused by the high vitamin K content of the seaweed. Seaweed may diminish the anticoagulant effect of vitamin K antagonists. Monitor therapy.(Bartle 2001, Daugherty 2006)

Adverse Reactions

Minor gastric adverse events have been reported in some clinical studies, including soft feces and feelings of fullness after 4 to 6 g doses of dried seaweed.Teas 2009

Seaweed is a rich source of iodine. Increased serum TSH levels have been demonstrated experimentally, and goiters have been reported in Japan where seaweed is widely consumed. The goiters appear to be only cosmetics and elimination of seaweed from the diet generally leads to shrinkage or disappearance of the goiter. Iodine concentrations in seaweed, as well as in dietary supplements, vary widely. In some cases, the United States maximum safe iodine intake of 1,000 mcg/day could be exceeded easily. Whether this would prove harmful is difficult to ascertain because susceptibility to the effects of a high intake of iodine appears to vary among individuals.Miyai 2008, Teas 2007 However, a case of maternal excess iodine intake that led to transient neonatal hypothyroidism has been reported in a Korean girl. Following almost daily consumption of seaweed soup, the infant’s elevated TSH, jaundice, and markedly elevated urine iodine was considered possibly related to elevated iodine in the mother’s breast milk. Thyroid function tests normalized within a few months of the mother ceasing to eat seaweed soup.Hamby 2018 Another case of iodine-induced hypothyroidism was reported in a 13-year-old Japanese girl with anorexia nervosa. Her total daily iodine intake was 34,700 mcg/day that resulted from daily intake of Kombu kelp (20 g/day), Japanese kelp and its broth. Her thyroid function improved within a week of stopping the Kombu kelp.Kita 2019

Daily intake of kelp resulted in a false-positive elevation in serum beta-D-glucan in a 55-year-old Japanese male recipient of a bone marrow transplant. The elevation was confirmed by a different assay and the patient’s beta-D-glucan levels normalized within 2 days of discontinuing his kelp consumption.Hashimoto 2017


Excessive intake of dried seaweed has been reported to cause carotenodermia (yellowing of the skin). Hypercarotenemia is caused by excessive intake of carotene-rich vegetables or drinks.Nishimura 1998 Arsenic is considered a human carcinogen and certain forms have been found in large amounts in seafood and seaweed; however, increases in urinary arsenic excretion have not been demonstrated in human studies.Hsueh 2002

Certain blue-green algae found in the Pacific, Indian, and Caribbean oceans have been associated with contact dermatitis. An isolate from the algae, debromoaplysiatoxin, is a very potent inflammatory agent, producing a follicular, papular, and pustular reaction in minute concentrations.Izumi 1987 Two other seaweeds, Gracilaria coronopifolia and Gracilaria tsudai, usually are considered nontoxic, but occasionally may produce poisons associated with GI symptoms when ingested. Extreme cases have resulted in death.Marshall 1998

Index Terms

  • Brown algae
  • Chlorophyta
  • Green algae
  • Phaeophyta
  • Red algae
  • Rhodophyta



This information relates to an herbal, vitamin, mineral or other dietary supplement. This product has not been reviewed by the FDA to determine whether it is safe or effective and is not subject to the quality standards and safety information collection standards that are applicable to most prescription drugs. This information should not be used to decide whether or not to take this product. This information does not endorse this product as safe, effective, or approved for treating any patient or health condition. This is only a brief summary of general information about this product. It does NOT include all information about the possible uses, directions, warnings, precautions, interactions, adverse effects, or risks that may apply to this product. This information is not specific medical advice and does not replace information you receive from your health care provider. You should talk with your health care provider for complete information about the risks and benefits of using this product.

This product may adversely interact with certain health and medical conditions, other prescription and over-the-counter drugs, foods, or other dietary supplements. This product may be unsafe when used before surgery or other medical procedures. It is important to fully inform your doctor about the herbal, vitamins, mineral or any other supplements you are taking before any kind of surgery or medical procedure. With the exception of certain products that are generally recognized as safe in normal quantities, including use of folic acid and prenatal vitamins during pregnancy, this product has not been sufficiently studied to determine whether it is safe to use during pregnancy or nursing or by persons younger than 2 years of age.

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