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Scientific Name(s):Over 9,000 seaweed species exist. Notable genera include Ascophyllum , Chondrus , Ecklonia , Fucus , Gelidium , Gracilaria , Laminaria , Pterocladia , and the taxa Phaeophyeota and Rhodophyceae .

Common Name(s): Seaweed , brown seaweed , red seaweed , algae , kelp , Irish moss/carrageenin , egg wrack , kombu/konbu , sea spaghetti , wakame , nori , dulse/dillisk , sea lettuce , sea grass


Clinical trials are generally lacking to support definitive therapeutic recommendations for seaweeds. However, seaweeds are an important nutritional source of minerals and elements and are low in sodium. Applications may exist for use in cardiovascular conditions due to potential in cholesterol reduction and appetite suppression. Alginates extracted from seaweed have been used in wound dressings.

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Clinical trials have used an oral dosage range of 4 to 12 g seaweed daily for up to 2 months.


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.


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. 1 While blue-green species, such as spirulina (cyanophyta), are called algae, they are actually photosynthetic bacteria. For information on spirulina, see the individual 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. 2 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. 2 , 3 , 4


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. 1 , 5

Uses and Pharmacology

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 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. 6 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. 7 , 8


Evidence for a preventive effect of seaweed on breast cancer is derived largely from epidemiological studies. 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. 9 , 10

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. 10 , 11 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. 12 Decreases in serum cholesterol and blood pressure are associated with daily consumption of seaweed either as powder or fiber. 5

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. 13

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. 5

In a crossover study, a seaweed-derived sodium alginate preparation decreased the mean energy intake, leading researchers to consider its contribution to obesity. 14

Cervical ripening

See individual Laminaria monograph .

Other effects

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. 15 , 16


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. 17


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. 18 , 19


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. 20 , 21


Clinical trials have evaluated the following dosages.

Reduction of cholesterol

5 g/day seaweed powder. 12


12 g seaweed per day. 13

Metabolic syndrome

4 to 6 g dried seaweed daily for 2 months. 5

Effects on estrogen

5 g/day seaweed for 7 weeks. 10


2,400 mg/day as a mineral supplement for 12 weeks. 20 , 21


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. 10 , 11 An increase in menstrual cycle length has also been demonstrated with the brown kelp, Fucus vesiculosus . 11


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. 22 , 23

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. 10

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. 24 , 25


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. 26 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. 27

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. 28 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. 29


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