Herb-Drug Interactions: Kelp

Fucus vesiculosus L. and other species (Fucaceae)

Synonym(s) and related species

Black tang, Bladder fucus, Bladderwrack, Cutweed, Kelpware, Rockweed, Seawrack.

Fucus serratus L., (known as Toothed wrack). Ascophyllum nodosum (L.) Le Jobs (known as Knotted wrack) is also used in a similar way to Fucus species.

Technically, kelps are species of Laminaria and Macro-cystis.


Kelp (British Ph 2009, European Ph, 6th ed., 2008 and Supplements 6.1, 6.2, 6.3 and 6.4).


The thallus of kelp contains polysaccharides including alginic acid (the major component), fucoidan and laminarin (sulfated polysaccharide esters), free phloroglucinol and its high-molecular-weight polymers the phlorotannins and fucols and galactobpids. The iodine content can be high, and kelp may be standardised to the total iodine content. Kelp also contains vitamins and minerals, particularly ascorbic acid (vitamin C), and it is a moderate source of vitamin K1 (phytomenadione). Kelp may be contaminated with various heavy metals such as arsenic, and it may be standardised to a maximum limit of these.

Use and indications

Traditionally kelp has been used as a source of minerals such as iodine for thyroid deficiency. It has also been used as a slimming supplement. Note that the iodine content in kelp may precipitate hyperthyroidism, and prolonged or excessive intake is inadvisable.


No relevant pharmacokinetic data found.

Interactions overview

Kelp is probably unlikely to interact with warfarin, because, although it is a moderate source of vitamin Kl5 and therefore has the potential to reduce the effect of warfarin and related anticoagulants, sufficient vitamin K is very unlikely to be attained with usual doses of kelp supplements. Note also that anticoagulant fucoidans in kelp are unlikely to be orally active.

Kelp + Anticoagulants

Unintentional and unwanted antagonism of warfarin occurred in one patient when she ate seaweed sushi. It has been suggested that kelp contains substances with anticoagulant activity, but the evidence for this is theoretical.

Clinical evidence

An isolated report describes a patient taking warfarin who had, on two occasions, reduced INRs of 1.6 and 1.8 (usual range 2 to 3) within 24 hours of eating sushi with seaweed (asakusa-nori). It was estimated that she had consumed only about 45 micrograms of vitamin K1, which would not usually be sufficient to interact. However, if her vitamin K stores were low, this amount could have accounted for a large percentage of her vitamin K intake or stores, and might therefore have interacted. Note that kelp is a moderate source of vitamin K, having about 66 micrograms per 100 g.However, this means that supplements containing 1 g of kelp will contain very little vitamin K (0.66 micrograms). Also, when the kelp is used to prepare an infusion, it would be unlikely to contain much vitamin K1; because the vitamin is not water soluble.

Experimental evidence

In experimental studies, fucoidans from brown seaweeds including kelp have demonstrated anticoagulant activity. For example, in one in vitro study, the fucoidan from Fucus serratus had anticoagulant activity, as measured by activated partial thromboplastin time; this was roughly equivalent to 19units of heparin permg. The fucoidin inhibited thrombin-induced human platelet activation. The fucoidans from Fucus vesiculosus and Ascophyllum nodosum had a smaller effect (roughly equivalent to 9 and 13 units of heparin per mg, respectively).


Kelp is a moderate source of vitamin K, and therefore, if eaten in sufficient quantities, would antagonise the effects of coumarins and indanediones, because these anticoagulants act as vitamin K antagonists. Fucoidans from kelp may act like heparin and inhibit thrombin activity, and therefore have some anticoagulant effects. However, they are large polysaccharides, and are therefore unlikely to be orally active.

Importance and management

The interaction of warfarin with vitamin K from foods is a very well-established, well-documented and clinically important drug-food interaction, expected to occur with every coumarin or indanedione anticoagulant because they have a common mode of action. However, the evidence suggests that, in patients with normal vitamin K1 status, in general, clinically relevant changes in coagulation status require large continued changes in intake of vitamin K1 from foods, which would be highly unlikely to be attained from usual doses of kelp supplements. This interaction would therefore be more applicable to kelp eaten as a food.

Fucoids in kelp are very unlikely to be orally active, so kelp supplements would be unlikely to have any anticoagulant activity.

Taking the evidence together, there appears to be no reason why patients taking warfarin should particularly avoid taking kelp supplements.

Kelp + Food

No interactions found.

Kelp + Herbal medicines

No interactions found.