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Glossary

Arsenic in supplements: inorganic vs organic forms

Written by the Full Measure editorial team, which sells supplements. Approved for publication by the Full Measure owners. Updated .

Drafted with AI assistance from the sources listed below. How we write these

Quick answer

Arsenic is a natural element in soil and water that can show up in supplements, especially those made from seaweed or other algae. Inorganic arsenic is the more harmful form. California's Proposition 65 safe harbor for it is 10 µg a day, but a total arsenic result does not say how much is inorganic.1,2,3

Key facts

  • In chemistry, organic arsenic means arsenic bound to carbon. It has nothing to do with organic farming.1
  • FDA tests foods for total arsenic first, then for the separate organic and inorganic forms when needed.1
  • In 33 algae and cyanobacteria supplements, total arsenic ranged from 0.053 to 57 mg/kg, with the highest levels in brown algae.2
  • Contaminated drinking water is the most common source of high inorganic arsenic exposure, according to the World Health Organization and CDC as cited by FDA.1
  • FDA's limit for arsenic in bottled water is 10 parts per billion, the same as EPA's for public drinking water.1

Inorganic and organic arsenic

Arsenic exists in several chemical forms. FDA describes inorganic arsenic as more dangerous than organic arsenic, with more severe health effects.1

Organic does not mean harmless. FDA says two organic forms found in some foods, DMA and MMA, may be a health concern and is following the research. In seaweed and algae supplements, much of the arsenic is in arsenosugars, which one study counted as potentially toxic while calling for more data.1,2

Where arsenic in supplements comes from

Arsenic is higher in some soils and rocks, near mining and some industry, and where arsenic pesticides were once used on crops. It can also enter during processing, for example through contaminated water.1

Seaweed and other algae stand out. In the 33 supplement study, inorganic arsenic ranged from below 0.02 to 4.7 mg/kg. In a 2007 report from California, eight of nine kelp supplements bought from local stores had arsenic above a 0.5 to 2 ppm FDA tolerance used for certain foods.2,4

Rice is another known food source. FDA has sampled rice and rice products for inorganic arsenic and set an action level for infant rice cereal.1

Reading an arsenic result

A total arsenic figure includes every form. If the total is low, the inorganic part must be low too. If the total is high, only a speciation test shows how much of it is inorganic.1

California's safe harbor applies to inorganic arsenic taken by mouth. Because inorganic arsenic is part of the total, a total under 10 µg per daily serving also keeps the inorganic part under the safe harbor.3,5

Health effects

FDA links long-term exposure to inorganic arsenic in adults with skin disorders, cancers of the skin, bladder and lung, and cardiovascular disease. Very high short-term exposure can cause nausea, vomiting, bruising and numbness or burning in the hands and feet.1

Exposure before birth and in infancy and childhood is of most concern for brain development, according to FDA.1

In a 2007 case report, a woman taking kelp supplements daily had raised urine arsenic along with hair loss, memory loss, rash, fatigue, nausea and vomiting, which resolved within weeks of stopping. A sample of her supplement held 8.5 mg/kg of arsenic.4

Questions people ask

Is organic arsenic toxic?

It is generally less harmful than inorganic arsenic, but not always harmless. FDA flags two organic forms, DMA and MMA, as possible concerns, and researchers want more data on the arsenosugars in algae.1,2

Why do kelp supplements contain arsenic?

Seaweed is a known source of arsenic, according to FDA, and brown algae such as kelp had the highest levels in a study of algae based supplements.2,6

Are there arsenic limits for supplements?

FDA has action levels for some foods, such as apple juice and infant rice cereal, and a 10 ppb limit for bottled water. Under its supplement manufacturing rule, each maker sets its own contaminant limits.1,7

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Sources

  1. U.S. Food and Drug Administration. Arsenic in Food. Content current as of March 5, 2024. Opened 2026-10-07.
  2. Cheyns K, Demaegdt H, Waegeneers N, Ruttens A. Intake of food supplements based on algae or cyanobacteria may pose a health risk due to elevated concentrations of arsenic species. Food Addit Contam Part A. 2021;38(4):609-621. Opened 2026-10-07.
  3. Office of Environmental Health Hazard Assessment. Proposition 65 No Significant Risk Levels (NSRLs) for Carcinogens and Maximum Allowable Dose Levels (MADLs) for Chemicals Causing Reproductive Toxicity. March 2021. Opened 2026-10-07.
  4. Amster E, Tiwary A, Schenker MB. Case report: potential arsenic toxicosis secondary to herbal kelp supplement. Environ Health Perspect. 2007;115(4):606-608. Opened 2026-10-07.
  5. Office of Environmental Health Hazard Assessment. Regulations on Proposition 65, California Code of Regulations, Title 27, Division 4, Chapter 1, with Appendix A, Proposition 65: A Summary. Unofficial copy, updated February 2025. Opened 2026-10-07.
  6. U.S. Food and Drug Administration. Cadmium in Food and Foodwares. Content current as of March 5, 2024. Opened 2026-10-07.
  7. 21 CFR 111.70. What specifications must you establish? Opened 2026-10-07.

This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.