ARE SOME SEAWEED POISONOUS?
No. There are no poisonous seaweeds in British waters, and very few anywhere in the world. Seaweed is unusual among wild food in this respect — there's no seaweed equivalent of the death cap, and misidentifying one species for another won't hurt you.
That doesn't mean everything on the shore is worth eating, or that foraging carries no risk at all. But the real risks are about where you pick and how much you eat, not which species you pick.
Species People Ask About
Desmarestia (acid weed, sea sorrel) — the one most often described as dangerous. It stores sulphuric acid in its tissues and turns sour as soon as it starts breaking down out of the water. It won't poison you, but it tastes foul and will spoil anything harvested alongside it. Foragers avoid it for practical reasons, not toxicity.
Wireweed (Sargassum muticum) — an invasive species along the British coast, and often listed as one to avoid. It's edible. Older fronds are tough and not much of a prize, but young growth is eaten in parts of East Asia, and sargassum more broadly is a food crop elsewhere.
Dead Man's Fingers (Codium spp.) — the name does it no favours. It's eaten regularly in Korea and Japan, where it's known as cheonggak and used in kimchi and salads. Not toxic, not off-limits, just unfamiliar in Britain.
Bladderwrack (Fucus vesiculosus) — safe and widely eaten. Like all seaweed, it takes up what's in the water around it, which is an argument about harvest site rather than about the species.
The Risks That Actually Matter
Where it grew. Seaweed absorbs its surroundings. Avoid harbours, marinas, sewage or industrial outflows, and anywhere with obvious run-off. Open coast with good water movement is what you want.
Iodine, in kelps. Brown seaweeds — particularly oar weed and other kelps — can be very high in iodine, and sustained heavy consumption can affect thyroid function. Occasional culinary use is not a concern. Eating large quantities of dried kelp daily is a different matter, and anyone with a thyroid condition should take medical advice before adding seaweed regularly to their diet.
Heavy metals. Seaweed can accumulate arsenic, cadmium and lead from the water. Hijiki in particular is high in inorganic arsenic and is subject to consumption advisories in several countries. Again: source and quantity.
Freshness. Harvest living seaweed attached to rock, not the material washed up on the strandline. Cast seaweed decomposes fast and you don't know how long it's been there.
Elsewhere in the World
A small number of tropical and subtropical species are genuinely toxic — some Caulerpa species contain caulerpin, and there are records of poisoning linked to Gracilaria in Pacific waters. None of these grow wild around Britain, and none are likely to be encountered by a UK forager.
A Note on This Page
An earlier version of this page described several British seaweeds as poisonous. That was wrong, and we're grateful to the forager who wrote in to tell us so. Seaweed is one of the safest things you can gather from a British shore.
Related
See our guide to edible seaweed in the UK for the species worth looking for, or the full guide to edible seaweed for the wider picture.
Seaweed has been a source of savoury flavour for centuries. Sozyë's sauces are brewed from a blend of wild Scottish seaweeds, harvested off the north coast of Scotland.