# Bladderwrack (Fucus vesiculosus)

*Sea wrack; *Quercus marina**

**Known today as.** *Fucus vesiculosus*, a brown seaweed of rocky shores, buoyed by paired air bladders.

Kind: Plant

> **Caution.** Its iodine can upset the thyroid.

## 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.

## 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 and toxicology

- **iodine**: variable, up to several hundred micrograms a gram
- **fucoidan**: a sulfated polysaccharide
- **alginic acid**: 

**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.

**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.

## 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.”

## 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.”
- Pliny the Elder, *Natural History* 26.66, trans. J. Bostock & H. T. Riley (1855), Perseus Digital Library.
- Grieve, M. (1931). *A Modern Herbal*, “Bladderwrack.” botanical.com/botanical/mgmh/b/bladde54.html
- Eliason, B. C. (1998). “Transient hyperthyroidism in a patient taking dietary supplements containing kelp.” *Journal of the American Board of Family Practice* 11.

---

*Bladderwrack.* In *Materia Medica*, edited by B. C. P. Statema. Version 1, revised 2026-10-01. The Thinking Press. https://thethinkingpress.org/materia-medica/bladderwrack

Historical description, not medical advice.