Is there a downside to taking krill oil?

When considering any supplement, even one as frequently praised for its heart and brain benefits as krill oil, a balanced view requires a close look at the potential drawbacks. While krill oil offers omega-3 fatty acids, primarily EPA and DHA, bound in phospholipids—a structure some suggest is more easily absorbed by the body than the triglycerides found in many fish oils—this benefit doesn't negate every possible negative effect or risk. ^[3][6] The reality is that while generally considered safe for most people when taken at recommended doses, there are distinct areas where krill oil consumption requires caution, planning, or outright avoidance.

# Digestive Effects

The most frequently reported downside associated with krill oil, mirroring issues seen with fish oil, centers on the digestive system. ^[5][10] Many users report experiencing gastrointestinal side effects, which can certainly detract from the desire to take the supplement daily.

These common complaints include belching, often accompanied by a noticeable fishy aftertaste or "fish burps". ^[1][2][5] For some individuals, this is manageable, but for others, it can be significant enough to cause them to stop supplementation entirely. Other gastrointestinal distress symptoms noted are heartburn, nausea, and generalized stomach upset. ^[2][10] In some cases, especially when higher doses are used or when the supplement is taken on an empty stomach, individuals may experience loose stools or diarrhea. ^[2][10]

It is worth noting that the phospholipid structure of krill oil is sometimes credited with reducing the incidence of these digestive issues compared to traditional fish oil supplements. ^[3] However, since the sources of these effects can vary between individuals, this benefit is not universal. A practical approach for those who experience this common hurdle is to start with a lower dose and ensure the supplement is taken alongside a meal containing fats. Splitting the daily dose between morning and evening may also help the digestive tract process the oil more smoothly, rather than being hit with the full amount at once.

# Shellfish Allergy Risk

A clear and significant contraindication for krill oil involves allergies. Krill are small crustaceans, meaning that anyone with a known allergy to shellfish—including shrimp, crab, or lobster—should strictly avoid krill oil supplements. ^[2][5][9]

The risk here is that an allergic reaction could occur even if the oil has been highly purified. While manufacturers take steps to remove proteins that typically trigger allergies, there is always a residual risk when dealing with an extract derived from a common allergen source. ^[5] For someone with a confirmed shellfish allergy, the potential severity of a reaction far outweighs any potential benefit from the omega-3s, making avoidance the only safe route. If you have any history of reactions to crustaceans, it is essential to discuss this with your healthcare provider before starting krill oil. ^[2]

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# Blood Thinning Potential

One of the recognized therapeutic properties of omega-3 fatty acids, whether sourced from fish or krill, is their ability to inhibit platelet aggregation, which effectively translates to a mild blood-thinning effect. ^[4][5] While this is often beneficial for cardiovascular health, it introduces a significant downside when interacting with certain medications or procedures.

This anticoagulant effect means that krill oil should be approached with great caution if you are already taking prescription blood thinners. Medications such as warfarin (Coumadin), aspirin, clopidogrel (Plavix), or even common nonsteroidal anti-inflammatory drugs (NSAIDs) taken regularly can have their effects amplified by the addition of krill oil. ^[2][5] The combination increases the risk of bruising and bleeding. ^[4]

Similarly, if you are scheduled for any surgery, including dental procedures, it is typically recommended to discontinue krill oil use at least two weeks prior. ^[5] This allows the body’s natural clotting mechanisms to return to baseline levels before a procedure where controlled bleeding is necessary. Medical professionals need full disclosure of all supplements being taken to adjust prescriptions or advise on safe cessation timelines. ^[2]

# Quality and Oxidation Concerns

While krill oil is often marketed as being more stable and less prone to oxidation (rancidity) than standard fish oil due to its phospholipid binding, ^[3] this does not render it immune to degradation. Oxidation is a major concern with any fat-based supplement because rancid oils not only taste bad but can also introduce free radicals into the body, potentially negating the intended health benefits. ^[6]

The difference in stability stems from the chemical structure: phospholipids allow the omega-3s to be incorporated more easily into cell membranes, which might offer some inherent protection against oxidation compared to the triglyceride form. ^[3] However, exposure to heat, light, and air during processing, storage, and transport remains a threat. ^[6]

Here is a critical point for consumers: the perceived stability advantage of krill oil is only maintained if the product is stored correctly after purchase. A poor-quality product, regardless of its starting structure, will eventually go rancid. Consumers should look for third-party testing seals, check the "manufactured on" date rather than just the "expiration" date, and always store the capsules tightly sealed in a cool, dark place, often the refrigerator, especially after opening the bottle. If your krill oil smells strongly fishy or "off," it should be discarded, as its chemical integrity is compromised. ^[6]

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# Practical Economic Consideration

While not a direct physiological side effect, the cost difference between krill oil and fish oil represents a very real downside for budget-conscious consumers, especially those needing high doses of omega-3s for therapeutic reasons. Generally, krill oil is significantly more expensive per dose than comparable fish oil supplements. ^[3][6]

When you compare the actual milligram dose of EPA and DHA—the active components—krill oil often requires a higher price tag. For instance, if a person needs 1,000 mg of combined EPA/DHA daily for a specific condition, the cost for that dose via a high-quality, purified fish oil might be substantially lower than the cost via krill oil. ^[6] This disparity means that for long-term use, krill oil can become a considerable financial commitment. Understanding that both sources deliver the same fundamental omega-3s allows a person to make an informed trade-off: paying a premium for the potential bioavailability edge of the phospholipid structure, or opting for the more economical triglyceride source. ^[3]

# Use in Specific Populations

There are specific groups of people for whom the downsides of krill oil are related more to a lack of established safety data rather than confirmed adverse events.

# Pregnancy and Breastfeeding

Data on the safety of krill oil specifically during pregnancy and while breastfeeding is limited. ^[5] While omega-3s are vital for fetal and infant development, most medical advice defaults to caution when an established safety profile is missing. Healthcare providers usually recommend sticking to well-studied sources, often involving dietary intake or fish oil supplements where more extensive data exists, rather than introducing a less-studied alternative like krill oil during these sensitive periods. ^[5]

# Pre-existing Conditions

Beyond the bleeding risk, individuals with pre-existing conditions need to exercise caution. Those with known bleeding disorders or who are scheduled for operations must be particularly mindful of the blood-thinning properties. ^[5] Furthermore, because krill oil affects blood lipids, anyone being treated for high cholesterol or high triglycerides should monitor their levels closely if they introduce krill oil, ensuring it supports, rather than interferes with, their prescribed medical management plan. ^[4]

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# The Importance of Dosage Consistency

While not a downside of the substance itself, a downside in practice arises from inconsistent dosing. Krill oil's benefits are cumulative, meaning they build up in the body's tissues over time. Missing doses or taking inconsistent amounts means the body never reaches the therapeutic threshold required for noticeable effects on inflammation or lipid profiles. ^[4] Conversely, frequently exceeding the recommended dosage, often in an attempt to "speed up" results, increases the likelihood of encountering those digestive side effects mentioned earlier. ^[2] Finding the minimum effective dose that remains tolerable—the intersection point between efficacy and comfort—is key to long-term, successful supplementation, which requires patience and adherence to guidelines rather than simply taking more. ^[4]

# Potential for Drug Interactions

The interaction profile for krill oil is less documented than that for fish oil, but the mechanism of action is similar enough that the same cautions apply, particularly concerning medications that affect blood clotting. ^[2][5] If a patient is taking high doses of vitamin E, for example, or any anticoagulant, the interaction risk is elevated. While krill oil is often marketed as a natural product, it is still a biologically active substance that can interfere with pharmaceutical action. Anyone taking prescription medication should treat krill oil as a supplement that requires consultation with a pharmacist or physician to review the complete medication list before initiation. ^[2]