Bladderwrack
Sea wrack; Quercus marina
- Known today as
- Fucus vesiculosus, a brown seaweed of rocky shores, buoyed by paired air bladders.
- Kind
- Plant, in 1 older authority
- Constituents
- iodine, fucoidan, alginic acid
- Earliest authority here
- Pliny (AD 23/24–79)
- In other databases
- Plants of the World Online search · GBIF search · Wikidata search
Revised October 1, 2026: An older authority added from a text read in full. Lore added from texts read in full. Version 1, first published September 30, 2026.
Caution. Its iodine can upset the thyroid.
Bladderwrack: what was it used for?
For swellings of the neck and glands, and later for obesity; burnt, its ash was an old remedy for goitre.
Bladderwrack: what does modern science say?
Iodine was discovered in 1811 in the ash of seaweed, which explains its effect on goitre. Too much iodine also harms the thyroid, and the plant can carry heavy metals.
Pharmacognosy & Toxicology
- Constituents
- iodine ChEMBLvariable, up to several hundred micrograms a gram
- fucoidana sulfated polysaccharide
- alginic acid ChEMBL
- Mechanism
- Iodine is taken up by the thyroid, which is why the ash cured goitre; too much shuts the gland down or drives it to overactivity.
- Toxic dose
- The iodine of a few grams a day exceeds the safe upper limit; arsenic and heavy metals accumulate in the weed.
- Standing
- No approved use; the thyroid cases are in the literature.
- Eliason, B. C. (1998). “Transient hyperthyroidism in a patient taking dietary supplements containing kelp.” Journal of the American Board of Family Practice 11. Verified against its record · doi:10.3122/jabfm.11.6.478
Constituent names open the compound’s record at PubChem, and “ChEMBL” beside a name opens its record at ChEMBL, the European Bioinformatics Institute’s database of bioactive molecules. Where a mechanism line was checked against ChEMBL (ChEMBL 37, read 1 October 2026), the check is shown with the measured value and its source paper, or says plainly that nothing has been measured. Doses are as the references give them, for description only; this is not medical advice. Each source carries the result of its check against Crossref, PubMed, or the authority’s page: see Verification.
Bladderwrack: what lore surrounds it?
Grieve records the black wrack “much used in Norway for feeding cattle, being called there ‘cowweed,’” and Linnaeus's Gothlanders who boiled it with meal for their hogs, “for which reason they called the plant ‘Swine-tang.’ In Sweden the poor people covered their cottages with it and sometimes used it for fuel.”
Bladderwrack: what did the older authorities say?
- Pliny
- “It is the phycos thalassion, or sea-weed, more particularly, that is so excellent a remedy for the gout. It resembles the lettuce in appearance, and is used as the basis in dyeing tissues with the purple of the murex. Used before it becomes dry, it is efficacious as a topical application not only for gout, but for all diseases of the joints.” Pliny distinguishes three kinds, “one with a broad leaf, another with a longer leaf of a reddish hue, and a third with a crisped leaf, and used in Crete for dyeing cloths,” and notes Nicander prescribing them in wine against the venom of serpents.
Sources
- Grieve, M. (1931). A Modern Herbal, “Bladderwrack.” Book, not indexed
- Pliny the Elder, Natural History 26.66, trans. J. Bostock & H. T. Riley (1855), Perseus Digital Library. Primary text
- Grieve, M. (1931). A Modern Herbal, “Bladderwrack.” botanical.com/botanical/mgmh/b/bladde54.html Standard reference work
Verification
- Checked
- October 1, 2026
- Passes
- Every sentence read against its sources, in the pass over the remaining 343 entries
- The Standing line checked against the authority it names (EMA, WHO, FDA, CITES, Commission E)
- Every dated or regulatory claim checked by category across the database
- Each quotation from the older authorities checked word for word against the text, read in full
- Sources
- 4 cited: 1 verified against its record, 1 primary text, 1 standard reference work, 1 book, not indexed.
- Corrections
- None were needed.
What was checked, how, and what was found is set out on the Verification page. A checked source is one whose record was found; it does not make the entry medical advice.
Cite This Entry
Version 1 · first published September 30, 2026 · last revised October 1, 2026
- Chicago
“Bladderwrack.” In Materia Medica, edited by B. C. P. Statema. Version 1. The Thinking Press, 2026–. Last modified October 1, 2026. https://thethinkingpress.org/materia-medica/bladderwrack.
- MLA
“Bladderwrack.” Materia Medica, edited by B. C. P. Statema, version 1, The Thinking Press, 1 Oct. 2026, thethinkingpress.org/materia-medica/bladderwrack. Accessed 8 Oct. 2026.
- APA
Bladderwrack. (2026, October 1). In B. C. P. Statema (Ed.), Materia medica (Version 1). The Thinking Press. Retrieved October 8, 2026, from https://thethinkingpress.org/materia-medica/bladderwrack
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See Also
Also in Pliny the Elder
Bloodstone in Pliny the Elder · Black pepper in Pliny the Elder · Broom in Pliny the Elder · Bitter aloes in Pliny the Elder · everything Pliny the Elder names →
Drawing on the same sources: Grieve
Nearby in the alphabet
Blessed ashes (blessed on Ash Wednesday) · Blackthorn (Prunus spinosa) · Blessed bread (distinct from the Eucharist) · Black walnut · Blessed candles (and the Paschal candle) · Blessed herbs (especially on the feast of the Assumption (15 August)) · Black hellebore (Helleborus niger)
Historical description, not medical advice.