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Cod Liver Oil

Also known as: fish liver oil, CLO, omega-3 fish oil, EPA/DHA supplement, vitamin A/D supplement

Evidence under review. — Not yet rated

Fish oil rich in omega-3s and vitamins A & D. Limited clinical evidence for most popular health claims.

  • What it does

    Cod liver oil is an oil extracted from the livers of cod fish, naturally rich in omega-3 fatty acids (EPA and DHA), vitamin A, and vitamin D. In a large clinical trial of over 34,000 adults, daily...

  • Evidence quality

    Evidence base hasn't been formally rated yet. See research below.

  • Clinical dose

    5 mL/day (liquid) or equivalent capsule dose based on study data

See full breakdown ↓ Methodology →

What the Science Says

Cod liver oil is an oil extracted from the livers of cod fish, naturally rich in omega-3 fatty acids (EPA and DHA), vitamin A, and vitamin D. In a large clinical trial of over 34,000 adults, daily supplementation showed no benefit for preventing COVID-19 or respiratory infections. A smaller trial in gestational diabetes patients found it reduced blood glucose, inflammation markers (hs-CRP), and insulin resistance after 12–16 weeks of use. Animal and lab studies suggest its omega-3 content may have anti-inflammatory and potential anti-tumor effects, but these findings have not been confirmed in human clinical trials.

What It Doesn't Do

Won't prevent COVID-19 or respiratory infections — a massive 34,000-person trial found zero benefit. No proven benefit for psoriasis on its own (it was only used as a vehicle in a topical ointment in one study). No human evidence it prevents Alzheimer's disease or improves brain health. Not a cancer treatment — animal data only. Don't expect it to replace a balanced diet or prescribed medications.

Evidence-Based Benefits

Reduces blood glucose, insulin resistance, and inflammation in gestational diabetes patients over 12–16 weeks.

Weak Evidence

Effective at: Cod liver oil providing ~76 mg/mL EPA and 150 mg/mL DHA daily

Does NOT reduce the risk of COVID-19 infection or serious illness — a large RCT found no benefit.

Weak Evidence

Effective at: 5 mL/day (10 µg vitamin D)

Mercury levels in commercial cod liver oil supplements are generally low and within safe limits.

Weak Evidence

Effective at: Standard supplement doses

Absorption & Bioavailability

Good — omega-3 fatty acids from fish oil are well absorbed when taken with food; fat-soluble vitamins A and D are absorbed alongside dietary fat. No specific bioavailability data provided in the supplied studies.

Red Flags to Watch For

  • Mercury contamination is a real concern with fish-derived oils — always choose products tested for heavy metals
  • Contains preformed vitamin A (retinol), which can be toxic at high doses — do not combine with other vitamin A supplements or high-dose multivitamins
  • Not effective for preventing viral respiratory infections despite popular belief — a large RCT found no benefit
  • Animal and in vitro cancer studies cannot be extrapolated to human cancer treatment — do not use as a cancer therapy
  • Quality varies widely across brands — mercury levels in tested supplements ranged significantly, though most were within safe limits

Research Sources

  • PubMed
  • NIH DSLD

This information is for educational purposes only and is not medical advice. Always consult a healthcare professional before starting any supplement regimen. Last updated: 2026-05-25