What are the Most Common Kelp Side Effects?

Eric Stolze

Kelp, or Fucus vesiculosus, is a brown-colored seaweed that grows along coastal areas of the northern Pacific and northern Atlantic oceans as well as the Baltic Sea and North Sea. This natural sea plant also has several other names including bladderwrack, black tang and rockweed. Natural kelp supplements and products are used by many people to improve their health and self-treat several medical conditions. These kelp supplements are typically prepared with extracts of Fucus vesiculosus and other related types of seaweed and may cause side effects in some people. The most common side effects may be linked to kelp’s iodine content and generally include increased or decreased thyroid hormone levels in the blood as well as severe skin lesions.

Kelp  is normally consumed in soups and shredded in salads.
Kelp is normally consumed in soups and shredded in salads.

Unwanted kelp side effects may vary from person to person, and many individuals consume kelp products without experiencing any side effects at all. The presence of iodine can cause stomach irritation, excessive saliva production or an unusual brass-like taste in the mouth. Some kelp products may be contaminated with heavy metals that can cause side effects related to these toxins. People who consume kelp contaminated by arsenic can experience nerve problems or kidney damage in some instances. Long-term use may cause a laxative effect in some people from alginic acid, a component of the seaweed.

Kelp's high iodine content can sometimes cause harmful interactions in the body.
Kelp's high iodine content can sometimes cause harmful interactions in the body.

Some medical professionals may recommend that pregnant women and breastfeeding mothers refrain from using kelp products due to their high iodine content and potential heavy metal contamination. Iodine in kelp can interact with thyroid medications such as levothyroxine and may alter the functioning of a patient’s thyroid in some cases. People who take kelp along with blood-thinning drugs such as warfarin or anti-platelet drugs such as clopidogrel may experience an increased risk of bleeding. Individuals who use laxative products together with kelp may notice an increased laxative effect in some cases.

Kelp can alter the functioning of the thyroid gland.
Kelp can alter the functioning of the thyroid gland.

The kelp plant is a common traditional food in Vietnam and some other Asian cultures. Raw kelp may be served as a vegetable or prepared in a salad, although it can be pickled or cooked in a vegetable soup. Many people have used kelp as a cosmetic and as a traditional medicine for several ailments such as coughs, stomachaches and hemorrhoids. Scientific evidence is generally inconclusive regarding the effectiveness of kelp for most medicinal uses.

In some case, kelp may fight bacterial or fungal infections due to its antibacterial and antifungal properties. Fucoidans are components of kelp that may have antioxidant effects in the human body. Fucans, another kelp component, and fucoidans may both have anticoagulant or blood-thinning effects in some people. In certain instances, kelp may lower levels of blood sugar in patients with diabetes.

Kelp may also have cancer-fighting properties that limit the growth of some cancer cells. Many products have high levels of iodine that can improve goiters in some people. A goiter is an enlarged thyroid gland that may react to changes in a person’s thyroid hormone levels and increased iodine consumption. Some weight-loss products include kelp, but the effectiveness of this seaweed for losing weight has not generally been studied by scientific researchers.

The iodine in kelp can sometimes cause stomach irritation.
The iodine in kelp can sometimes cause stomach irritation.

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Discussion Comments


I have been taking two kelp tablets daily for about three weeks. My metabolism seems to have been boosted and I feel more energetic also I have reduced my laxatives to one tablet daily as the kelp has a laxative effect on me so for me that is a positive.


I have been using a kelp supplement daily -- one tablet. I am feeling great and very energetic. I had a sluggish thyroid, but after using kelp and with a diet change, my thyroid is getting back to normal. I completely stopped eating cruciferous vegetables like cabbage, broccoli, peas and cauliflower. I engage in normal exercise. We have to change our lifestyle in order to keep our thyroids healthy. Good luck and take care. --Mujahid


I've been using kelp for years. I have lost 9 stone in weight. I was a big, big lady. I have had no side effects. at all. In fact, I never felt better. That was until the darn menopause took over.


I started taking Kelp tablets once a day for thyroid support. Dosage 650 mcg. After taking for about two weeks, I am having stomach upset, cramps and loose bowel. Otherwise, I am well, no flu or cold, so I have to suspect my symptoms are related to the kelp.

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