Evidence Based This post has 22 references

Is Cod Liver Oil Good For You? Side Effects & Safety Data

Written by Joe Cohen | Last updated:
Carlos Tello
Puya Yazdi
Medically reviewed by
Carlos Tello, PhD (Molecular Biology), Puya Yazdi, MD | Written by Joe Cohen | Last updated:
Cod Liver Oil Safety Data Are Conflicting

Cod liver oil is a popular all-around health supplement, but just how safe is it? Analyses indicate that fermented cod liver oil might be rancid. Some data also suggest that high doses of regular cod liver oil may lead to vitamin A overdose and vitamin E deficiency. Get the research facts in this post.

Cod Liver Oil Side Effects & Precautions

A high intake of cod liver oil has been associated with some health risks. Because the link was often found in cohort studies, this doesn’t necessarily mean that cod liver oil caused these conditions.

However, caution is advised, particularly in sensitive populations (children, pregnant women, the elderly, and critically-ill).

Additionally, cod liver oil supplements have not been approved by the FDA for medical use. Supplements generally lack solid clinical research. Regulations set manufacturing standards for them but don’t guarantee that they’re safe or effective. Speak with your doctor before supplementing.

1) Vitamin A and D Ratio Imbalance

Some researchers oppose the use of cod liver oil due to a risk of vitamin A overdose with vitamin D deficiency. While this ratio imbalance may explain the discrepancy between data on cod liver oil’s effect on bone mineral density, the vitamin A content in cod liver oil was recently reduced by 75% in Norway [1].

2) Thyroid Cancer Concerns

A study of 60,000 Norwegians associated the consumption of fish and cod liver oil with an increased incidence of thyroid cancer. Since thyroid follicle cells are sensitive to vitamin D, the improper ratio of vitamin A to D is a possible factor [2, 3].

3) High Blood Pressure During Pregnancy

A study of 549 pregnant women associated slightly higher intakes of the omega-3 fatty acids found in cod liver oil (more than 0.87 g/day or 1 tsp/5 mL of cod liver oil) in the early stages of pregnancy with an increased incidence of high blood pressure. The dosage associated with the lowest risk was 0.10 – 0.87 g/day (or 0.6-5.0 mL of cod liver oil) [4].

4) Bone Weakness

A study of over 3,000 women in Norway associated cod liver oil intake during childhood with lower bone mineral density. The authors speculated that a decrease in vitamin A levels in cod liver oil may prevent this effect, as high vitamin A level increases the risk of bone fractures [5, 6, 7, 8].

However, multiple studies came to opposite findings. This might be due to a recent reduction in vitamin A levels in cod liver oil, which may help restore the balance between vitamin A and vitamin D [6, 9, 10].

Limited studies suggested an association between cod liver oil intake and pregnancy complications, thyroid cancer, and bone weakness. More research is needed.

5) Potentially Increased Bleeding Time

The effects of cod liver oil on blood clotting are inconclusive. Some studies found this oil increased bleeding time, while others failed to observe this effect [11, 12, 13, 14, 15].

High doses of cod liver oil may increase the risk of excessive bleeding, vitamin A overdose, and vitamin D deficiency. However, the data remain inconclusive and safety seems to vary among products depending on their vitamin levels.

Drug Interactions

To help avoid interactions, your doctor should manage all of your medications carefully. Be sure to tell your doctor about all medications, vitamins, or herbs you’re taking. Talk to your healthcare provider to find out how cod liver oil might interact with something else you are taking.

Cod liver oil could interact with the following drugs and supplements:

  • Blood thinners: Cod liver oil may increase bleeding time so it might be dangerous to consume with blood-thinning medications [11, 12, 13].
  • Vitamin E: Excess levels of cod liver oil can cause vitamin E deficiency [16].

The Dosage of Cod Liver Oil Determines Its Safety

Doses Used Safely

Cod liver oil is likely safe when taken appropriately by mouth.

The following daily doses have been safely used in clinical trials:

  • 10 mL for up to 24 weeks
  • 15 mL for 16 weeks
  • 20 mL to 30 mL for 2 weeks to 8 weeks

Vitamin RDAs and Tolerable Upper Intakes

Since cod liver oil is a source of vitamins A and D, the recommended daily allowances (RDAs) of these vitamins should not be exceeded. The amount of cod liver oil this amounts to may vary from one product to the next.

The RDAs of these two vitamins are [17, 18]:

  • Vitamin E: 900 mcg (3000 units of retinol) for men and 700 mcg (2333 units of retinol) for women
  • Vitamin D: 15 mcg (600 units) for all adults up to 70 years of age and 20 mcg (800 units) daily for older adults

Cod liver oil is also likely safe in children, and breastfeeding and pregnant women if vitamin A and D intakes are within the RDAs.

Clinical trials report safely using 2.5 mL of cod liver oil daily in children aged 6 months to 1 year and 5 mL daily in children aged 1 year to 5 years for 5 months [19].

According to one clinical trial, a daily dose of 10 mL of cod liver oil (containing vitamin A 1170 mcg and vitamin D) by mouth was safe from week 17 of pregnancy, through delivery, and or 3 months during breastfeeding [20].

Taking doses of cod liver oil that contain more than the Tolerable Upper Intake Levels (ULs) of vitamins A and D is likely unsafe. This would amount to [17, 18] :

  • Vitamin A: 3000 mcg (10,000 units of retinol) daily for adults and 2800 mcg (9333 units of retinol) daily for adolescents aged 14 years to 18 years.
  • Vitamin D: 100 mcg (4000 units) daily for everyone 9 years of age and older

Additionally, higher cod liver oil doses (20-40 mL per day) have been linked with prolonged bleeding time in healthy volunteers. Larger trials should investigate this further.

Vitamin A and D RDAs should not be exceeded with cod liver oil supplements in any population. Check the label and consult your healthcare provider if unsure.

Is Fermented Cod Liver Oil Safe?

Fermented vs. Extra Virgin Cod Liver Oil

Fermented and extra virgin both refer to the method of extracting the oil from the cod livers. Fermented cod liver oil is extracted by fermenting cod livers and collecting the extruded oil. Extra virgin cod liver oil is extracted from raw cod liver without using heat.

While many tout fermented cod liver oil as superior to non-fermented oil, others claim fermented cod liver is not fermented at all but decomposed (and is, thus, rancid and maybe even putrefied). Neither of these claims has been substantiated. Fermented cod liver oil has not been extensively studied, unlike non-fermented cod liver oil.

There is no consensus on whether fermentation of oil can occur, whether fermented cod liver oil is rancid, and how to most accurately test for the rancidity of fermented cod liver oil.

It’s uncertain what fermented cod liver oil is. Manufacturers claim to make it by extracting oil from fermented cod livers, but others claim these products are not fermented but putrefied and rancid. Unbiased, high-quality analyses and studies haven’t yet been carried out.

The Importance of Quality Testing

If you decide to supplement, it’s important to buy cod liver oil from trusted brands. Manufacturers of high-quality cod liver oil supplements provide detailed third-party test results on parameters such as vitamins and fatty acid content, dioxins and PCBs, and heavy metals. Data on the rancidity and oxidation by-products of the supplement should also be made available.

A scandal in 2015 brought attention to one popular brand called Green Pastures that didn’t meet these standards, misleading consumers, and potentially putting them at a significant health risk.

This came as a surprise, as proponents previously claimed that Green Pastures produced the only truly “traditional” fermented cod liver oil. It quickly became a favorite in the paleo community, despite its undoubtedly bad, burning taste and dark color.

Dr. Kaayla Daniel detailed the results of her independent “underground” analysis of Green Pastures Fermented Cod Liver Oil (commonly branded as Blue Ice). Due to the nature of the experiment, her results have not been published in the scientific literature but are available online. She claims to have run an unbiased scientific analysis in her search for the truth behind popular fermented cod liver oil claims [21].

On the other hand, there is no way to know how she controlled for some factors as part of her analyses, who did the analyses (this is marked as confidential data), and how accurate and adequate the chosen tests were to identify certain cod liver oil quality parameters.

In 2015, Dr. Kaayla Daniel conducted a “secret” analysis of Green Pastures Fermented Cod Liver Oil, suggesting that its contents don’t match the label. Although informative, her analysis has major limitations and her results haven’t been published in peer-reviewed journals.

With this in mind, let’s take a look at what she discovered.

The Green Pastures Fermented Cod Liver Oil Scandal

Here’s we’ll aim to sum up the 100-page report that details Dr. Kaayla Daniel’s analyses [21].

Fermented or Not?

First of all, the analysis indicated that Green Pastures Fermented Cod Liver Oil probably wasn’t fermented at all. Here’s why [21]:

  • Lactic acid fermentation requires sugars and other carbs, which cod liver oil doesn’t contain
  • The oil’s pH was only slightly acidic (5.2-5.8), less so than that of typical fermented foods (below 4.6)
  • It contained far less lactic acid bacteria that fresh, fermented foods (10 colony-forming units or CFU opposed to 1-50 million CFU)
  • Since the oil likely wasn’t fermented, it wasn’t preserved either and is potentially unsafe (fermentation preserves food by creating an acidic environment that bacteria can’t survive in)
  • There is no evidence that the oil was “fermented,” that is, preserved, in any other way (e.g. by sun-drying, algae, storing the fish in the earth for a couple of days, or submerging it in brine)
Limited analyses suggest that Green Pastures Cod Liver Oil is not fermented.

Vitamin Levels and Rancidity

Next, Green Pastures Fermented Cod Liver contained significantly lower levels of fat-soluble vitamins than cod liver oil should. Plus, the report suggests that it probably doesn’t contain vitamin D2 but only contains D3, which is typical of cod liver oil (unlike the company claimed) [21].

Additionally, the product contained very low levels of all forms of vitamin K. Specifically, its vitamin K2 levels were 17-19 ng/g. That’s in nanograms per gram, equivalent to about 1.7 micrograms/100 g. Chicken, which is not among the best K2 sources, contains about 14-32 micrograms/100g [22].

The supplement also had undetectable CoQ10 levels.

The most concerning discovery? This product seemed to be rancid. Analyses showed the following:

  • High peroxidase and free fatty acid levels in the oil suggest extreme rancidity, which may be responsible for its “disagreeable” taste (but more accurate tests are needed to confirm this)
  • Typical rancidity markers of the oil seem to be acceptable, but this type of testing likely only helps identify short-term rancidity. This fermented cod liver oil, on the other hand, is supposed to be a product of “long-term fermentation”

These results should be taken with a grain of salt, though. As mentioned, free fatty acid and peroxidase levels are not an accurate measure of rancidity in fish oils. More reliable and sensitive tests are needed.

Another concerning fact is that the supplement appeared to contain about 3% trans fats. Cod liver oil, by all standards, should not contain trans fats. This means that their product may have been adulterated with another, unknown oil (such as processed vegetable oil).

Lastly, the data on the levels of biogenic amines were conflicting.

Green Pastures Cod Liver Oil seemed to contain much lower levels of fat-soluble vitamins. Contradictory analyses suggested extreme rancidity, but this is questionable since it’s still uncertain how long-term rancidity should be tested for.

Is it Cod At All?

Dr. Kaayla Daniel’s analyses indicated that Green Pastures Fermented Cod Liver might not be made from cod at all. Its vitamin levels, omega-3 fatty acid ratio, and DNA profile closely matched Alaskan pollock, not cod.

For example, arctic cod liver should contain far less EPA than DHA (the EPA:DHA ratio is usually 6:10 or 9:14). Analyses reported on the Green Pastures website and those of independent testing companies are close enough and point to an exceptionally high EPA to DHA ratio (13.5:6.5 and 16.2:7.4).

If this wasn’t enough, Dr. Kaayla Daniel claims that, based on DNA analysis showed, the liver is “100 percent Alaskan pollock.”

One DNA analysis indicated that Green Pastures Cod Liver Oil may, in fact, be Alaskan pollock.

What Next?

Although valuable, this investigation opens more questions than it answers.

We need more unbiased, in-depth research about the safety of fermented and fresh cod liver oil. When it comes to fermented cod liver oil, high-quality scientific data are especially lacking. Both lab-based and human studies are needed.


Raw cod liver oil is likely safe when taken by mouth at about 10 to 15 mL daily.

Higher doses of raw cod liver oil may cause vitamin A overdose, vitamin D deficiency, and possibly excessive bleeding.

No valid scientific data are available on fermented cod liver oil. Until adequate studies are carried out, the safety of fermented cod liver oil remains unknown.

If you decide to supplement, be sure to consult your healthcare provider and ask manufacturers for quality certificates.

Read Next

About the Author


1 Star2 Stars3 Stars4 Stars5 Stars
(1 votes, average: 5.00 out of 5)

FDA Compliance

The information on this website has not been evaluated by the Food & Drug Administration or any other medical body. We do not aim to diagnose, treat, cure or prevent any illness or disease. Information is shared for educational purposes only. You must consult your doctor before acting on any content on this website, especially if you are pregnant, nursing, taking medication, or have a medical condition.

Leave a Reply

Your email address will not be published. Required fields are marked *

Related Articles View All