Sea Moss Side Effects: What to Check First (UK Guide)

The main side effect risk with sea moss comes from iodine: seaweeds soak up iodine from the sea in amounts that vary with species, harvest site and season, and too much iodine can make the thyroid overactive or underactive. The other things to check first are heavy metals, which seaweeds also take up from the water, and whether a medicine you take or a condition you have rules seaweed out.

Key takeaways

• Iodine matters most. Adults need 140 micrograms a day, and the upper level is 600 micrograms a day from all sources. Seaweed iodine varies, and too much can push the thyroid either way.
• Thyroid conditions and some medicines mean a doctor first. That includes levothyroxine, lithium, amiodarone and any other iodine-containing medicine.
• Heavy metals are the second check. Look for third-party lab testing for heavy metals, and do not eat hijiki seaweed, which the Food Standards Agency advises against.
• Not for pregnancy, breastfeeding or under-18s. Our bottle contains burdock root, so it is not for anyone allergic to daisy-family plants.
• Headaches, blood pressure and kidneys: we found no sea moss trials. Persistently high iodine can cause headaches, and anyone with high blood pressure, heart disease or kidney disease should ask a GP first.

Sea moss side effects at a glance

We could not find a published randomised controlled trial of Irish sea moss in humans, so most of what follows comes from research on iodine, seaweed in general and bladderwrack, the brown seaweed often sold with it.

France's food safety agency, ANSES, says persistently high iodine intake can cause diarrhoea and headaches. Some people say sea moss leaves them bloated. We could not find a clinical study that measured bloating, so we cannot say how common it is. If it happens to you, stop taking it.

Iodine: the main side effect to check

Your thyroid uses iodine to make its hormones. The NHS says adults need 140 micrograms a day, and food labels use a reference figure of 150 micrograms. The upper level for adults, set by the EU's Scientific Committee on Food, is 600 micrograms a day from food, drink and supplements together.

The issue with seaweed is that its iodine is hard to predict:

• In an Italian study of 14 seaweeds and algae sold for eating, iodine ranged from 3.4 to 7,316 milligrams per kilogram of dried weight.
• Irish research found seaweeds generally hold more iodine in winter than summer.
• In a 2015 survey of 224 seaweed products on sale in the UK, only 10% stated their iodine content, and 26 could potentially take someone over the 600 microgram upper level.

So the British Dietetic Association says seaweed is not a reliable way to make sure you get enough iodine, and ANSES says seaweed in supplements is not appropriate for correcting a lack of iodine. If you need a guaranteed iodine intake to a set number, the right tool for that job is a supplement that declares its iodine content in micrograms, not a seaweed.

In practice: stick to the serving on the label and count other iodine sources, such as a multivitamin with iodine, kelp tablets or seaweed snacks. Vegan and relying on seaweed for iodine? Read Vegan Iodine Sources: Why It Matters and What to Eat.

Sea moss and your thyroid

A 2014 review in Nature Reviews Endocrinology names the people at higher risk from too much iodine: people with an existing thyroid condition, older people, and unborn and newborn babies. In them, excess iodine can cause an underactive or an overactive thyroid, sometimes visible only on a blood test.

In a UK trial, 42 women with low iodine took capsules of a brown seaweed daily for two weeks. Their TSH, the hormone doctors use to check the thyroid, rose, and two went above the normal range (their thyroxine stayed normal). In a case report, a 45-year-old woman with no previous thyroid problem followed a kelp-containing diet product. Her thyroid was overactive for two months, then underactive, and she needed three months of levothyroxine before it recovered.

Medicines. The European Medicines Agency (EMA) says bladderwrack medicines should not be taken with iodine preparations or thyroid medicines, and records an overactive thyroid in a man taking lithium. ANSES advises anyone on medicines containing iodine or lithium to seek medical advice before taking seaweed products, and notes that dietary iodine can change how amiodarone, a heart rhythm medicine, affects the thyroid. Any of these means a doctor first. If you are due radioactive iodine therapy, follow your hospital's low-iodine advice.

Signs to look out for. The NHS lists anxiety, trouble sleeping, a fast or irregular heartbeat, trembling and weight loss among the symptoms of an overactive thyroid, and tiredness, feeling cold, weight gain, constipation and low mood for an underactive one. All have other causes, but if any appear after you start a seaweed product, stop and see your GP. More in Sea Moss and Thyroid Function: The Iodine Connection.

Heavy metals: what seaweed picks up from the water

Seaweeds concentrate what is in the water around them, minerals and contaminants alike. Iodine, arsenic, cadmium, lead and mercury all vary with species and harvest site.

• In the Italian study above, total arsenic reached 67.6 milligrams per kilogram, cadmium 7.2 and lead 6.7. The authors did not see a threat at typical intakes, but said these levels should be controlled.
• Arsenosugars, organic forms of arsenic, are the most common type found in seaweed. Hijiki is different: the Food Standards Agency advises people not to eat it, because a survey found high levels of a form of arsenic known to have carcinogenic (cancer-causing) properties. Hijiki is not one of our ingredients, and the testing point below applies to every seaweed, ours included.
• A 2026 Cornell University study of 66 sea moss products bought online in the US detected arsenic, cobalt and mercury in the dried sea moss, and bacteria, including Salmonella, in some products.

When you compare products, look for third-party lab testing that covers heavy metals, every ingredient named with an amount in milligrams, and a maker you can identify and trace, which ANSES also recommends. Our Sea Moss Gummies are third-party lab tested.

Bladderwrack and burdock root: the other two ingredients

Bladderwrack is a brown seaweed, and the British Dietetic Association notes that brown seaweeds in particular have a very high iodine content, so everything above applies to it. For bladderwrack medicines, the EMA does not recommend use by under-18s or in pregnancy or breastfeeding. Its assessment report lists high blood pressure, kidney disease and anaemia as reasons for caution, and possible interactions with blood pressure medicines and blood thinners as well as lithium and thyroid medicines. It also recommends limiting how long bladderwrack medicines are taken and checking thyroid function regularly, for example every four months, so if you take any seaweed product for months at a time, ask your GP whether a thyroid blood test is worth doing.

Burdock root comes from the daisy family (Asteraceae), alongside ragweed, chamomile and chrysanthemums. The EMA's monograph says burdock root medicines should not be used by anyone allergic to that family and records anaphylaxis, a severe allergic reaction, at an unknown frequency. It also advises against taking them with prescribed diuretics (water tablets). If your lips, mouth or throat suddenly swell, or you struggle to breathe, call 999.

Sea moss side effects for women and men

We found no research comparing sea moss side effects in women and men.

Pregnancy and breastfeeding. Do not take sea moss. LactMed, the US drugs and breastfeeding database, reports that excess iodine from seaweed eaten by breastfeeding mothers has suppressed their babies' thyroids. More in Sea Moss and Pregnancy: What to Know About Iodine and Safety.

Women. The NHS says Hashimoto's disease, the main cause of an underactive thyroid in the UK, is most common in women. A 2004 report on three women taking bladderwrack found their menstrual cycles lengthened by 5.5 to 14 days. Three people with no comparison group cannot show cause, but the EMA includes this report in the safety section of its bladderwrack monograph, so if your cycle changes, tell your GP.

Men. We found nothing specific to men, but the 2014 review lists older people among those at higher risk from excess iodine.

Sea moss, blood pressure and kidneys

We could not find any trial of sea moss on blood pressure or kidney function, so there is no evidence it is good for either. There are reasons to check first:

• ANSES says persistently high iodine intake can, less commonly, cause heart problems, and the EMA notes that bladderwrack from polluted water can carry arsenic or heavy metals that may damage the kidneys.
• ANSES advises people with heart disease or kidney failure not to take seaweed products without medical advice.
• Seaweeds naturally contain potassium, and the NHS says people with chronic kidney disease can develop high potassium levels.

If you have high blood pressure, heart disease or kidney disease, or take medicine for any of them, speak to your GP before taking sea moss.

Who should not take sea moss

• Anyone pregnant or breastfeeding. Do not take it.
• Anyone under 18. Our bottle is for adults.
• Anyone allergic to seaweed, to any ingredient on the label, or to daisy-family plants. Do not take it.
• Anyone with a thyroid condition, on thyroid medicine, or due radioactive iodine therapy. Speak to your doctor first.
• Anyone taking lithium, amiodarone or any other iodine-containing medicine. Speak to your doctor first.
• Anyone taking blood thinners, blood pressure medicines, diuretics or medication for blood sugar. Speak to your doctor or pharmacist first.
• Anyone with heart disease, kidney disease or anaemia. Speak to your doctor first.
• Anyone already taking kelp, another seaweed supplement or an iodine supplement. Do not combine them without advice.

Food supplements are not a substitute for a varied diet and a healthy lifestyle.

What is in our Sea Moss bottle and how to take it

Sea moss and bladderwrack are seaweeds and carry iodine naturally, so the iodine and thyroid points above apply to this bottle.

Each bottle holds 60 gummies, a 30-day supply. They are pectin based and vegan, with no gelatine and no artificial colours or flavours.

How to take it: two gummies a day, any time, with or without food, and no more than two a day. See How Many Sea Moss Gummies Should You Take a Day?, and for gummies against homemade gel, Sea Moss Gel vs Gummies vs Capsules: A UK Guide.

Buying the Daily Three? Its Ashwagandha and Saffron bottles have their own cautions, covered in Can You Take Sea Moss, Ashwagandha and Saffron Together?

List prices, before any discount code, correct on 27 October 2026.

Frequently asked questions

Is sea moss good for you?

Sea moss has no authorised health claim of its own in the UK, and we could not find a published randomised controlled trial of Irish sea moss in people. Whether it suits you depends mainly on iodine, which seaweeds carry in varying amounts. It is not for pregnancy, breastfeeding or under-18s, and anyone with a thyroid, heart or kidney condition, or taking thyroid medicine, lithium or amiodarone, should speak to a doctor first.

Are sea moss gummies good for you?

Sea moss, bladderwrack and burdock root have no authorised health claims of their own in the UK. A gummy gives you a fixed weight of each ingredient, printed in milligrams, which homemade gel cannot, but the iodine in any seaweed still varies with species, harvest site and season. Before you buy any brand, check it is third-party lab tested for heavy metals, and do not take it alongside kelp, an iodine supplement or another seaweed product. Sea moss gummies are not for pregnancy, breastfeeding or under-18s, and anyone with a thyroid, heart or kidney condition, or taking thyroid medicine, lithium or amiodarone, should speak to a doctor first.

Can sea moss cause headaches?

We could not find any study of sea moss and headaches. France's food safety agency, ANSES, lists headaches and diarrhoea among the effects of a persistently high iodine intake, and seaweeds carry iodine in amounts that vary. If new headaches start after any seaweed product, stop taking it, and if they carry on, speak to your GP or a pharmacist.

Is sea moss good for your kidneys?

We could not find any trial of sea moss and kidney function, so there is no evidence that it is. France's food safety agency, ANSES, advises people with kidney failure not to take seaweed products without medical advice, and the European Medicines Agency lists kidney disease as a reason for caution with bladderwrack, a seaweed often sold with sea moss. Seaweeds also contain potassium, which can build up in chronic kidney disease. Ask your kidney team or GP first.

Is sea moss good for high blood pressure?

We could not find any trial of sea moss and blood pressure, so there is no evidence that it is. The European Medicines Agency lists high blood pressure as a reason for caution with bladderwrack, a seaweed often sold with sea moss, and France's food safety agency, ANSES, advises people with heart disease not to take seaweed products without medical advice. If you have high blood pressure or take medicine for it, ask your GP first.

Can you take sea moss with levothyroxine?

Speak to your doctor first. Sea moss and bladderwrack are seaweeds and carry iodine in varying amounts. The European Medicines Agency says bladderwrack medicines should not be taken with thyroid medicines, and the British Dietetic Association notes that with thyroid disease, an iodine intake that is normal for most people may still affect you. The same applies to lithium, amiodarone and other iodine-containing medicines.

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References

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Food supplements are not intended to treat, cure or prevent any disease. If you are pregnant, breastfeeding, taking medication, or have a medical condition, consult a healthcare professional before use.