Alginate, Agar and Carrageenan: How Seaweed Got Into Your Medicine Cabinet

By Jayson Byles, forager & chef · September 2026

Seaweed is in medicine already, and has been for longer than most people realise. Alginate from kelp dresses wounds and makes the raft in indigestion remedies, carrageenan and agar carry and set drugs, agar grows the cultures every hospital laboratory depends on, and fucoidan and carrageenan are being tested against viruses and as ways to deliver other drugs. This page is the medical half of the other uses of seaweed: what is established, what is research, and where a Scottish shore fits. We write it as harvesters who supply raw material, not as clinicians, and the line between the two is drawn plainly throughout.

What is alginate?

Alginate is the polysaccharide that gives brown seaweed its structure: a long chain of two sugar acids that the kelps and wracks build their cell walls from. Extracted with alkali and sold as sodium alginate, it dissolves in water into a viscous solution and sets into a soft gel the moment it meets calcium, at any temperature, without heat. That one property is behind almost every medical use on this page: the floating raft in a reflux remedy, the gel a wound dressing turns into, the beads that release a drug slowly, the dental impression that sets in the mouth. On our shore it is in oarweed, sugar kelp and the wracks; the chemistry, and its uses beyond medicine, are on our seaweed materials page.

Is Gaviscon really made from seaweed?

Yes, in part. The active ingredient that makes the difference between a plain antacid and a raft antacid is sodium alginate, extracted from brown seaweed. On contact with stomach acid the alginate forms a gel that floats on the stomach contents and sits in the way of reflux. That raft is why the product works, and its quality depends on the alginate, which in turn depends on the seaweed: a study in the International Journal of Pharmaceutics found raft strength varied with the seaweed source the alginate came from. The kelps we cut on the Fife shore, oarweed and sugar kelp, are the same genus the industry draws on, and the raft antacid was a British invention, made in Hull.

Alginate dressings: the seaweed that heals wounds

Alginate is the structural polysaccharide of every kelp and wrack. Spun into fibres and made into pads and ropes, it does something no cotton dressing can: it swaps the calcium in the fibre for the sodium in a wound’s fluid, and turns into a soft gel that holds moisture against the wound bed while drawing off the excess. Alginate was first reported in surgery in 1947 and modern dressings date from the early 1980s, when the British firm BritCair launched Kaltostat (history in the Journal of Tissue Viability). They are now standard in NHS formularies for exuding wounds, leg ulcers, pressure sores and donor sites. A 2025 review follows the newer generation, alginate blended with silver, honey, chitosan or growth factors, from bench to clinic. This is the mature end of seaweed medicine; nobody argues about the evidence, only about grade, purity and supply.

Carrageenan, agar and the excipients nobody notices

Most seaweed in medicine is invisible. Carrageenan from the red seaweed carrageen and its relatives is a pharmacopoeial excipient: it thickens suspensions, stabilises emulsions and forms the matrices that release a drug slowly over hours. Agar, from other red seaweeds, is the jelly in the Petri dish on which nearly all microbiology is done, and a gentle laxative in its own right. A 2023 review in Marine Drugs covers the pharmaceutical uses of the seaweed hydrocolloids together.

The question people actually search for is whether carrageenan is bad for you. The European Food Safety Authority re-evaluated it in 2018 and found no safety concern at the levels used in food, while asking for better data on degraded forms (EFSA, 2018). Food-grade carrageenan and the degraded “poligeenan” used to provoke inflammation in laboratory animals are different substances, and much of the worry online comes from confusing the two. Whole carrageen simmered into a pudding, the way it has been eaten in Ireland and Scotland for centuries, is neither.

Iodine: the first seaweed medicine

Iodine was discovered in 1811 by Bernard Courtois, a French saltpetre maker, when he treated the liquor from burnt seaweed ash with sulphuric acid and saw a violet vapour (Britannica). For most of the nineteenth century the world’s iodine came from kelp, and a good part of that kelp was burnt on Scottish shores; the history of seaweed in Scotland tells that story. Today iodine remains the one seaweed nutrient with authorised health claims, and the reason brown seaweeds carry warnings as well as praise: the NHS puts adult needs at 140 micrograms a day and says supplements of 0.5 mg or less a day are unlikely to cause harm (NHS), and the kelps can hold thousands of milligrams per kilogram dry.

What the research says now

The early-stage work is where the surprises are. Each of these is dated, sourced and graded, because the distance between a cell-culture result and a medicine is measured in years and in trials that mostly fail.

• Antiviral fucoidans (cell culture, 2024). Researchers at Mount Sinai and Rensselaer found that fucoidans and rhamnan sulphates from seaweed blocked SARS-CoV-2 entry in cell culture, in some cases more potently than the reference drug (study, 2024). It is a mechanism worth pursuing, not a treatment.
• Carrageenan nasal sprays (human trials). Iota-carrageenan sprays that coat the nasal lining were tested against COVID-19 in a UK-relevant protocol, ICE-COVID (protocol, 2022); the products already sell for the common cold in several countries.
• Fucoidan as an anticoagulant (animal, 2024). Fucoidan has heparin-like activity, and a 2024 study built it into dissolving microneedle patches that delivered it through skin in animal models (study).
• Alginate drug delivery (review, 2025). Sodium alginate beads, films and gels that release drugs in the gut, the eye or a wound are reviewed in AAPS PharmSciTech; several are in products, more are in development.
• Fucoidan nanocarriers (review, 2025). Beyond capsules, fucoidan is being built into nanoparticles that carry other drugs and target tumour cells, still at cell and animal stage (review).
• Fucoidan in a real trial (human, due 2027). The Mayo Clinic trial of fucoidan against chemotherapy-related fatigue is the kind of properly controlled study the compound has lacked. A 2026 review in Frontiers in Plant Science sets out where fucoidan and alginate stand in biomedicine as a whole.

What almost all of this needs first is a modest, reliable quantity of one identified species. That is the gap a small hand-harvester can fill, and it is where the applied-science work on seaweed materials and the medical work meet.

The Scottish thread

Scotland’s part in seaweed medicine runs from the kelp-iodine trade of the 1800s to the wartime agar hunt. In 1942, with Japan’s agar supply cut off and bacteriology laboratories short of culture medium, Sheina Marshall and Andrew Orr at the Millport marine station surveyed the Clyde shores and identified Gigartina stellata, a relative of carrageen, as a British source (SAMS). The Scottish Association for Marine Science in Oban is still the country’s seaweed research centre, and Marine Biopolymers in Ayr works on alginate and other extracts from Scottish seaweed. The Scottish Government’s 2022 sector study models bioactives as the fastest-growing segment of the industry (gov.scot). On our shore we cut the same kelps and the same carrageen; the difference between food and medicine is what happens after the harvest.

Food-grade, pharmaceutical-grade, and what the words mean

Food-grade seaweed is clean, identified and handled under food-hygiene rules. Pharmaceutical use needs a defined, tested raw material with documented species, batch and contaminant results, and the product made from it needs authorisation as a medicine or a device before it can be sold with a medical claim. The MHRA’s Guidance Note 8 sets out when a product becomes a medicine by presentation, and a seaweed food described as treating a condition crosses that line. We supply the raw material for both, and we make no medical claims for anything we sell. If your work is at the nutraceutical end, the compounds, the human trials and the novel-food rules are on the companion page: seaweed nutraceuticals and functional foods.

Getting seaweed for your project

We supply every species we harvest except laver: fresh, cut on the low tide and chilled the same day, in 5 kg lots; or dried in small bespoke batches as whole leaf, pieces or flakes. Every bag carries the species, the shore and the date. Batch testing and a certificate of analysis are available on request. We cut above the growing point and never pull, so the same bed is there next season, and we harvest under the licence rules explained in our guide to the law. For a first project, ask for a sample pack of what is in season and tell us what you are making.

Sources

The figures and findings above come from these sources; the reviews are open access where we could find them.

• Alginate raft formation and seaweed source (International Journal of Pharmaceutics, 2005)
• Alginate dressings: a history (Journal of Tissue Viability, 1997)
• Alginate-based wound dressings from preclinical to clinical (International Journal of Biological Macromolecules, 2025)
• Algal phycocolloids: bioactivities and pharmaceutical applications (Marine Drugs, 2023)
• Re-evaluation of carrageenan (E 407) as a food additive (EFSA, 2018)
• Bernard Courtois and the discovery of iodine (Britannica)
• Iodine (NHS)
• Fucoidans and rhamnan sulfates against SARS-CoV-2 (2024)
• ICE-COVID: iota-carrageenan nasal and throat spray trial protocol (2022)
• Fucoidan dissolving microneedles for transdermal anticoagulant delivery (2024)
• Sodium alginate in drug delivery systems (AAPS PharmSciTech, 2025)
• Fucoidan-based pharmaceutical formulations and nanocarriers (2025)
• Fucoidan for preventing chemotherapy-related fatigue, Mayo Clinic trial (ClinicalTrials.gov)
• Fucoidan and alginate from brown seaweeds: extraction, structure and biomedical applications (Frontiers in Plant Science, 2026)
• Seaweed at SAMS in wartime and today (Scottish Association for Marine Science)
• Understanding the potential scale for seaweed-based industries in Scotland (Scottish Government, 2022)
• Guidance Note 8: a guide to what is a medicinal product (MHRA)

Questions we get asked

What is alginate?

Alginate is the structural polysaccharide of brown seaweeds such as kelp and wrack. Sold as sodium alginate, it dissolves in water and sets into a soft gel on contact with calcium, which is why it is used as the raft in reflux remedies, in wound dressings, in slow-release tablets and in dental impression material.

Is Gaviscon made from seaweed?

Partly. Its raft-forming ingredient is sodium alginate, extracted from brown seaweed. In stomach acid the alginate sets into a gel that floats on the stomach contents and sits in the way of reflux, and studies show the strength of that raft varies with the seaweed the alginate came from.

What is an alginate dressing?

A wound dressing made from fibres of calcium alginate, a polysaccharide from kelp. In contact with wound fluid the calcium swaps for sodium and the fibre turns into a moist gel that protects the wound bed and absorbs exudate. They have been in clinical use since the early 1980s and are standard in NHS formularies.

Is carrageenan bad for you?

The European Food Safety Authority re-evaluated carrageenan in 2018 and found no safety concern at the levels used in food, while asking for better data on degraded forms. Food-grade carrageenan and the degraded form used in laboratory inflammation studies are different substances, and whole carrageen simmered into a pudding is neither.

Can seaweed cure viruses or cancer?

No. Fucoidans and carrageenans have shown antiviral activity in cell culture and carrageenan nasal sprays have been tested in people, but no seaweed compound is an approved treatment for a viral infection or a cancer. The results are reasons for trials, several of which are under way, not treatments.

This page describes how seaweed compounds are used and studied in medicine. It is not medical advice, and nothing we sell is a medicine. Talk to a pharmacist or doctor about any condition or treatment.