Common Gastrointestinal Side Effects
For many users, the most common side effects of fish oil are related to the digestive system and are generally mild. These discomforts often occur when taking high doses or consuming the supplement on an empty stomach.
- Fishy Burps (Eructation): One of the most frequently reported side effects is a fishy-tasting burp. This occurs when the oil lingers in the stomach and is released through gas bubbles.
- Nausea and Indigestion: Some individuals experience nausea, stomach upset, and heartburn, especially shortly after taking the supplement.
- Diarrhea: High doses of fish oil, often exceeding 3 grams per day, are linked to diarrhea in some individuals.
- Other GI Symptoms: Additional digestive issues can include abdominal pain, bloating, and gas.
To manage these issues, some strategies include freezing the capsules, taking them with a meal, or splitting the daily dose into smaller portions. Enteric-coated capsules are also available to prevent digestion until the oil reaches the small intestine, which can help reduce fishy burps.
Serious Health Risks and Warnings
While uncommon, certain severe side effects and interactions are associated with fish oil, particularly at high doses or in specific populations. Consulting a healthcare provider is essential to determine safety based on individual health history.
Increased Bleeding Risk
High doses of fish oil can have an antiplatelet effect, slowing down blood clotting. While many clinical trials suggest that this does not significantly increase bleeding risk in most people, it remains a concern for individuals with bleeding disorders or those taking blood-thinning medications like warfarin. A 2024 meta-analysis found no overall increased risk, but some very high-dose purified EPA supplements may have a modest association. Doctors may still recommend stopping fish oil before surgery as a precaution.
Low Blood Pressure (Hypotension)
Fish oil has been shown to have a modest blood pressure-lowering effect. For people with high blood pressure, this can be beneficial. However, for those with naturally low blood pressure or who are already taking blood pressure medication, adding fish oil could cause blood pressure to drop too low.
Atrial Fibrillation (AFib)
In some studies, very high-dose fish oil supplementation has been linked to a slightly increased risk of atrial fibrillation, an irregular heart rhythm. This risk appears higher in individuals with pre-existing cardiovascular conditions, especially with doses over 1 gram daily.
Potential for Vitamin A Toxicity
Certain fish oil supplements, like cod liver oil, contain high levels of vitamins A and D. Consuming too much over time can lead to vitamin A toxicity, with symptoms including dizziness, joint pain, skin irritation, and potentially liver damage.
Fish Oil vs. Cod Liver Oil: Side Effect Comparison
| Side Effect | Fish Oil (General) | Cod Liver Oil | Notes |
|---|---|---|---|
| Fishy Burps/Taste | Common, especially with low-quality or rancid oil. | Can occur, similar to other fish oils. | Can be mitigated by freezing capsules or taking with food. |
| Nausea/Indigestion | Common, particularly at high doses or on an empty stomach. | Common, same gastrointestinal issues as general fish oil. | Taking with meals can often alleviate this side effect. |
| Increased Bleeding | Possible at very high doses or when combined with blood thinners. | Possible at high doses, same mechanism. | Always consult a doctor if taking blood thinners. |
| Low Blood Pressure | Possible, especially with high doses. | Possible due to omega-3 content. | Monitor blood pressure closely if on medication. |
| Vitamin A Toxicity | Not a risk unless specifically containing vitamins A/D. | Significant risk due to high vitamin A content. | Monitor vitamin A intake to avoid liver damage. |
| Allergic Reactions | Possible for individuals with seafood allergies. | Possible for individuals with seafood allergies. | Seek medical help immediately if severe allergic symptoms occur. |
Conclusion: Navigating the Risks for Safer Supplementation
While fish oil offers notable health benefits, awareness of its potential side effects is paramount. Most common issues, such as digestive discomfort and fishy aftertaste, are mild and can often be managed by adjusting the dosage, timing, or type of supplement. More serious risks, including those related to bleeding, blood pressure, and atrial fibrillation, are typically associated with high doses and are more relevant for specific populations. Individuals taking other medications, especially blood thinners, must consult a healthcare professional to avoid adverse interactions. Ultimately, getting omega-3s from a diet rich in fatty fish is often recommended over relying solely on supplements. For those who choose supplementation, sourcing a high-quality product and adhering to recommended dosages is the safest approach.
Visit the NIH Office of Dietary Supplements for more information on omega-3s
Other Considerations
Fish oil's anti-inflammatory properties can be beneficial, but excessively high doses might also suppress the immune system over time. Purity is also a factor, as lower-quality supplements may contain contaminants like mercury, though reputable brands use purification processes. For many people, starting with a lower dose and taking it with food can significantly reduce the incidence of side effects, allowing for the benefits of omega-3s without the discomfort.
