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Glossary

Mercury in supplements, including fish oil

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

Most mercury people take in comes from fish and shellfish, as methylmercury. Studies of oil supplements have found low levels: five fish oils ranged from nondetectable to 12 µg/L, and 36 fish liver and vegetable oil supplements all stayed under a 0.10 mg/kg standard.1,2,3

Key facts

  • Methylmercury, the main form in seafood, is also the most harmful form of mercury, according to FDA.1
  • Nearly all fish and shellfish contain traces of methylmercury, and levels differ a lot between species.1
  • In the 36 oil supplements, vegetable oils averaged about twice as much mercury as cod liver oils, though all were low.3
  • California lists mercury and methylmercury compounds under Proposition 65, but OEHHA's safe harbor list gives no daily level for them.4,5
  • In a 2008 study, Ayurvedic products made by combining herbs with metals had a median mercury level of 20,800 µg per gram, against 34.5 µg in other Ayurvedic products.6

Where mercury comes from

Mercury is released by volcanoes and weathering rock, and by human activity such as burning fossil fuels, manufacturing and small scale gold mining. Fish and shellfish carry it as methylmercury, which is how most people in the U.S. are exposed.1

Fish oil starts from fish, so it is the obvious supplement to ask about. In one study, five over the counter brands ranged from below 6 µg/L, the detection limit, to 10 to 12 µg/L, similar to the level normally found in human blood.2

Which supplements to ask about

  • Fish oil, fish liver oil and other marine oils, because the raw material comes from fish.1
  • Traditional Ayurvedic rasa shastra preparations, which combine herbs with metals and had far more mercury than other Ayurvedic products in the 2008 study.6
  • Seaweed products, which FDA notes can carry mercury, though less of it than arsenic and cadmium.7
  • Vegetable oil supplements, which are not automatically lower: in one study they averaged more mercury than fish liver oils.3

Reading a mercury result

ICP-MS, the method FDA uses for several elements at once, gives total mercury. FDA's manual has a separate method for methylmercury, and the form matters because methylmercury is the more harmful one.1,8

There is no Proposition 65 safe harbor number for mercury, so a mercury result cannot be checked against one the way lead or cadmium can. For scale, FDA's limit for mercury in bottled water is 2 ppb.1,5

Health effects

High methylmercury exposure while the brain is developing can affect intelligence, memory, attention and motor skills, so FDA and EPA advice on eating fish focuses on pregnancy, breastfeeding and young children.1

In adults, harmful levels can cause numbness, muscle weakness, trouble with speech, hearing or vision, a metallic taste and poor balance. FDA says suspected poisoning should be diagnosed by a doctor through a blood or urine test.1

Questions people ask

Does fish oil contain mercury?

It can contain traces, but measured levels have been low. Five fish oil brands tested in one study had mercury similar to what is normally found in human blood.2

Is there a legal limit for mercury in supplements?

Under FDA's supplement rule, each maker sets its own contaminant limits. California lists mercury under Proposition 65 without a safe harbor level, and a Polish study judged oil supplements against a 0.10 mg/kg standard for dietary supplements.3,5,9

Is methylmercury the same as mercury?

It is one form of mercury, and the main form in seafood. FDA describes it as the most potentially harmful form.1

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Sources

  1. U.S. Food and Drug Administration. Mercury in Food. Content current as of March 5, 2024. Opened 2026-10-07.
  2. Foran SE, Flood JG, Lewandrowski KB. Measurement of mercury levels in concentrated over-the-counter fish oil preparations: is fish oil healthier than fish? Arch Pathol Lab Med. 2003;127(12):1603-1605. Opened 2026-10-07.
  3. Brodziak-Dopierała B, Fischer A, Chrzanowska M, Ahnert B. Mercury exposure from the consumption of dietary supplements containing vegetable, cod liver, and shark liver oils. Int J Environ Res Public Health. 2023;20(3):2129. Opened 2026-10-07.
  4. 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.
  5. 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.
  6. Saper RB, Phillips RS, Sehgal A, et al. Lead, mercury, and arsenic in US- and Indian-manufactured Ayurvedic medicines sold via the Internet. JAMA. 2008;300(8):915-923. Opened 2026-10-07.
  7. U.S. Food and Drug Administration. Cadmium in Food and Foodwares. Content current as of March 5, 2024. Opened 2026-10-07.
  8. U.S. Food and Drug Administration. Elemental Analysis Manual (EAM) for Food and Related Products. Opened 2026-10-07.
  9. 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.