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Herb library · Seaweed · Chondrus crispus

Sea moss (Irish moss)

A red seaweed of North Atlantic shores that sets into a jelly when boiled, sold today as a dried seaweed and a ready-made gel. No study has tested it in people. It carries iodine and can carry heavy metals, and a 2026 test of 66 US products found bacteria in both forms and arsenic and mercury in the dried seaweed.

Traditional use

Gathered on the west coast of Ireland and the Massachusetts coast, bleached in the sun, and boiled into a jelly. It entered medicine in Dublin in 1831 as a nourishing, soothing food, and was used in jellies, soups and blancmange. Today it is sold as a dried seaweed and ready-made gel.

What studies show

  • Insufficient

    Minerals, thyroid, immunity and other promoted uses: LactMed says no medicinal value has been proven, and no clinical study in people was found.

Be careful of

Cautions

  • AmountSea moss contains iodine, and seaweeds vary enormously in how much (ODS). The adult upper limit for iodine is 1,100 mcg a day; too much can cause goiter or thyroid over- or underactivity.
  • NoteIt can contain arsenic, lead, cadmium and mercury from the water it grew in (LactMed). A 2026 test of 66 US products found arsenic, cobalt and mercury in dried sea moss, and bacteria including Salmonella and Cronobacter in some products.
  • PregnancyLactMed advises avoiding sea moss while breastfeeding unless its iodine and heavy metal levels are known to be low: excess seaweed iodine eaten by mothers has suppressed the thyroid of breastfed babies.
  • InteractionHigh iodine intake with antithyroid drugs such as methimazole can cause hypothyroidism (ODS).
  • Ask a clinicianTalk with a clinician or pharmacist before using this if you take prescription medicines, are pregnant or breastfeeding, or are giving it to a child.

From the old books

What herbals and dispensaries of the past printed. History, in their words.

“It was introduced into medicine in 1831, by Todhunter, of Dublin. Carrageen (more properly carraigeen) signifies, in Irish, “moss of the rock””
Harvey Wickes Felter and John Uri Lloyd, King's American Dispensatory (1898), vol. 1, p. 525. See the page of King's American Dispensatory, vol. 1, p. 525 (opens in a new tab) · Read the full passage from King's American Dispensatory, vol. 1, p. 525

Sources

  1. Sea Moss (LactMed) (opens in a new tab) · NIH · read September 25, 2026
  2. Microbial and Heavy Metal Analyses of Commercially Available Sea Moss and Sea Moss Products (2026) (opens in a new tab) · NIH · read September 25, 2026
  3. Iodine: Fact Sheet for Health Professionals (opens in a new tab) · NIH ODS · read September 25, 2026

Reviewed September 25, 2026.