Sea moss (Irish moss)
Summary
No human trial has tested sea moss supplements for any health outcome, so its benefits are unproven (no grade). The main issue is safety: seaweed products vary widely in iodine, from 5 to 5,600 mcg per daily dose in one market survey, and can carry arsenic, lead, cadmium and mercury. Excess iodine can disrupt thyroid function.
Key facts
- A red seaweed (Chondrus crispus) used as food and folk medicine; contains carrageenan, iodine and minerals[1][2]
- None: we found no human trials of sea moss itself; one small trial of its carrageenan fibre in ulcerative colitis found earlier relapse (D)[1][2][3]
- No RDA for sea moss. Its iodine counts toward the adult iodine UL of 1,100 mcg/day[7]
- High iodine intake can add to the effect of antithyroid medicines such as methimazole[7]
- 1 graded human outcome on this page
- October 10, 2026
How it works
ProposedSea moss is mostly carbohydrate (carrageenan and other polysaccharides) plus minerals. Any effect would depend on its iodine, which the thyroid uses to make hormones, and on its fibre. No human mechanistic studies of sea moss were found, so claimed benefits remain proposed.[1][2][7]
What the human research says
Human trials only, one row per outcome. The form column shows what each trial actually tested. Grades follow our evidence-grading policy.
| Outcome | Effect | Grade | Best evidence | PMID |
|---|---|---|---|---|
| Relapse in ulcerative colitis with carrageenan | Worsened | Grade D | RCT · n = 12 · 2017 Adults with ulcerative colitis in remission on a no-carrageenan diet Form: carrageenan capsules (the main fibre in sea moss), not sea moss · Dose: Less than average daily dietary carrageenan intake (amount not stated in abstract) · Duration: Up to 1 year 3 people on carrageenan capsules relapsed vs none on placebo; IL-6 rose in the carrageenan group. | 28447072 (opens PubMed)Source [3] |
Forms
| Form | Elemental % | Absorption (human data) | GI tolerance | Studied for |
|---|---|---|---|---|
| Dried sea moss and homemade gel | Varies[1] | No human absorption data in our sources.[1] | No human tolerance data; iodine and heavy-metal content depend on where it was harvested.[1][2] | Folk and wellness use; no trials |
| Sea moss capsules and gummies | Varies[4] | No human absorption data in our sources.[1] | Seaweed supplements in one market survey ranged from 5 to 5,600 mcg iodine per daily dose.[4] | Wellness use; no trials |
Studied doses
Doses studied in human trials ranged from none at all: we found no human trial of sea moss supplements, so there is no evidence-based amount. Iodine from any seaweed counts toward the 1,100 mcg/day adult upper limit.[1][2][7]
Interactions
Published interactions only: each row cites a source. No row means none was found in our sources, not that a combination is safe.
Medicines
- Antithyroid medicinesNIH fact sheetHigh iodine intake with antithyroid medicines can have an additive effect and could cause hypothyroidism.[7]
Who should be careful
- Thyroid disease
- People with thyroid conditions are more sensitive to excess iodine. A case report describes hyperthyroidism in a woman with goiter after 4 weeks of kelp tea.[6][5]
- Breastfeeding
- The NIH LactMed database advises avoiding sea moss while breastfeeding unless iodine and heavy-metal levels are shown to be low; maternal seaweed iodine has suppressed infants' thyroids.[1]
- Pregnancy
- No safety studies in pregnancy; iodine needs are higher but excess iodine also carries thyroid risks.[7][6]
- Children
- No safety data for children in our sources; their iodine upper limits are lower than adults'.[7]
Talk to your pharmacist or clinician before starting a supplement, especially if you take prescription medicines.
Why sea moss is a safety-first page
The concern with sea moss is not proven benefit but uncertain content.
- Iodine varies by thousands of micrograms
- In 96 seaweed products tested in Norway, supplements ranged from 5 to 5,600 mcg iodine per daily dose, and whole seaweed from 128 to 62,400 mcg per portion.[4]
- Heavy metals
- Sea moss can contain arsenic, lead, cadmium and mercury from the water it grew in.[1][2]
- Excess iodine and the thyroid
- Excess iodine can trigger hyperthyroidism, hypothyroidism or goiter in susceptible people, especially those with existing thyroid disease.[6]
How to read the label
- Label dose vs clinical doseThere is no clinical dose: no human trial of sea moss exists in our sources. The weight of sea moss per serving on a label says nothing about the iodine it delivers.[1][4]
- Look for an iodine amount in mcgIf the Supplement Facts panel lists iodine, compare it with the 1,100 mcg/day adult upper limit, counting multivitamins and iodized salt too.[7]
- Ask for heavy-metal testingBecause seaweed can carry arsenic, lead, cadmium and mercury, a certificate of analysis from the seller is the only way to know levels for a batch.[1][2]
Frequently asked questions
Does sea moss have any proven benefits?
Not in humans. We found no clinical trials of sea moss supplements; NIH LactMed notes that no medicinal value has been proven.[1]
How much iodine is in sea moss?
It varies widely. Seaweed supplements in one survey ranged from 5 to 5,600 mcg per daily dose, against an adult upper limit of 1,100 mcg/day.[4][7]
Is sea moss safe for your thyroid?
Not reliably. Excess iodine can cause hyperthyroidism or hypothyroidism, especially in people with thyroid disease, and seaweed iodine content is unpredictable.[6][4]
Can I use sea moss while breastfeeding?
LactMed advises avoiding it unless iodine and heavy metals are shown to be low, because maternal seaweed iodine has suppressed infants' thyroid function.[1]
Does sea moss contain heavy metals?
It can: arsenic, lead, cadmium and mercury have been reported, depending on where it was harvested.[1][2]
Is carrageenan in sea moss bad for the gut?
One 12-person trial in ulcerative colitis found earlier relapse with carrageenan capsules. It is too small to judge whole sea moss, but people with inflammatory bowel disease may want to know.[3]
Sources
- [1]Sea Moss., 2006. database
- [2]Risks and benefits of consuming edible seaweeds.. Nutr Rev, 2019. review
- [3]A randomized trial of the effects of the no-carrageenan diet on ulcerative colitis disease activity.. Nutr Healthy Aging, 2017. RCT
- [4]Commercially available kelp and seaweed products - valuable iodine source or risk of excess intake?. Food Nutr Res, 2021. observational
- [5]Iodine-induced thyrotoxicosis after ingestion of kelp-containing tea.. J Gen Intern Med, 2006. observational
- [6]Excess iodine intake: sources, assessment, and effects on thyroid function.. Ann N Y Acad Sci, 2019. review
- [7]Iodine: Fact Sheet for Health Professionals. NIH Office of Dietary Supplements, 2024. fact-sheet
What changed
- First draft. Every PMID checked against NCBI E-utilities esummary (title and year taken from the record) and abstracts read for the cited rows.
- Adversarial review pass: every claim re-checked against the cited abstract (or full text where noted), PMIDs re-verified via NCBI esummary, unsupported label-market statements removed, two evidence rows with unreported pooled n removed. Page set indexable.