How long can you keep used fish oil?

November 22, 2025 •

4 min read

According to the USDA, properly strained and refrigerated used frying oil can be safely stored for up to three months. However, when it comes to used fish oil, the lifespan is often much shorter due to its high polyunsaturated fat content, which is prone to oxidation. Understanding proper storage and knowing the signs of spoilage is crucial for both flavor and health.

Quick Summary

Used fish oil has a much shorter shelf life than other cooking oils due to its rapid oxidation. Proper storage involves straining, using an airtight container, and refrigeration to prevent rancidity. Watch for visual and odor changes to determine if the oil is still safe to use. Never reuse oil from frying fish for subsequent savory or sweet dishes due to flavor transfer.

Key Points

Limited Reusability: Used oil from frying fish should be reused minimally, ideally only once for similar dishes, due to its high omega-3 fatty acid content that oxidizes rapidly.
Proper Storage is Key: Store strained, cooled oil in an airtight, opaque container and refrigerate it to slow down spoilage.
Trust Your Senses: Check for signs of rancidity like a foul or overly fishy smell, a dark or cloudy appearance, and a bitter taste before reusing.
Know the Risks: Consuming rancid oil can produce harmful free radicals and is linked to long-term health issues like inflammation and cardiovascular problems.
Dispose of It Safely: Never pour used oil down the drain. For small amounts, seal in a container and place in the trash. For larger volumes, check local recycling options.

Why Is Used Fish Oil Different from Other Cooking Oils?

Unlike vegetable or peanut oil, fish oil is packed with fragile polyunsaturated omega-3 fatty acids. This molecular structure, while healthy, makes it highly susceptible to a process called oxidation when exposed to heat, light, and air. This chemical reaction causes the oil to break down quickly, turning it rancid far faster than more stable saturated fats or monounsaturated oils.

Deep frying, in particular, exposes oil to extremely high temperatures that accelerate this degradation process. The small particles of food left behind from frying also hasten spoilage. A food scientist at EatingWell explained that repeatedly heating oil depletes beneficial fatty acids and creates harmful compounds, including aldehydes and trans fats. This means a "once-used" fish oil from deep frying has already undergone significant chemical changes and is fundamentally different from the fresh oil you started with.

How to Safely Store and Reuse Fried Fish Oil

Given the rapid degradation, the best practice is to avoid reusing oil specifically used for frying fish more than once, if at all. If you choose to reuse it, especially for a purpose where the flavor is not a concern (e.g., frying more fish), strict protocols are necessary.

Cool Completely: Always let the oil cool down to room temperature before handling. Pouring hot oil is extremely dangerous.
Strain Thoroughly: Strain the cooled oil through a fine-mesh sieve lined with cheesecloth or a coffee filter to remove all food particles and debris. This is a critical step, as leftover crumbs will cause the oil to spoil faster.
Choose the Right Container: Store the oil in an opaque, airtight container to protect it from light and air. The original bottle can work, but a glass jar or metal can is also suitable.
Refrigerate: For optimal freshness, refrigerate the container of used fish oil. The cold temperature significantly slows down the oxidation process. The oil may appear cloudy and solidify when cold, which is normal and will clear up when it returns to room temperature.
Label Clearly: Clearly label the container with the date and what was fried in it. This helps prevent flavor cross-contamination and keeps track of its age.

Signs That Used Fish Oil Has Gone Bad

Even with the best storage practices, used fish oil will eventually go rancid. Your senses are the best tool for checking if the oil is still good. Pay attention to:

Smell: A foul, pungent, or overly "fishy" odor is the most telling sign of rancidity. Fresh fish oil should have a mild, oceanic scent. If it smells unpleasant, toss it.
Appearance: Check for cloudiness or darkening that doesn't disappear when the oil is brought to room temperature. Excessive foaming or frothing when reheated is another red flag.
Flavor: If you take a tiny taste, rancid oil will have a sharp, bitter, or acrid flavor. A bad taste is a sure sign of spoilage. Flavors from previous cooking, particularly fish, will also persist.
Texture: The oil might become thick or sticky, indicating significant degradation.

Comparison: Storing Used Fish Frying Oil vs. Canned Fish Oil


Aspect	Used Fish Frying Oil	Canned Fish Oil
Source	Leftover oil from pan-frying or deep-frying fish.	Oil packed with tinned seafood like sardines, tuna, or anchovies.
Composition	Starts as a high-heat stable oil (like peanut or vegetable) but is altered by heat, air exposure, and food particles.	High-quality olive or other oil used to preserve the fish.
Storage Duration	No more than a few weeks in the refrigerator, and preferably only once or twice for flavor.	Up to 4 days refrigerated after opening, as it is considered a prepared food.
Flavor Impact	Imparts a strong fish flavor to subsequent foods fried in it.	Infused with the savory, delicious flavor of the tinned fish.
Reuse Application	Best for re-frying the same type of food, or discarded. Not suitable for delicate flavors like donuts.	Excellent for vinaigrettes, pasta salads, sauces, or sautéing vegetables.
Disposal	Small quantities should be sealed in a container and thrown in the trash. Large quantities may require specialized recycling.	Small quantities are usually consumed. Larger amounts can be disposed of in the trash.

The Health Risks of Using Rancid Oil

Consuming rancid fish oil is not just an unpleasant experience; it can pose potential health risks over time. The oxidation process creates harmful free radicals that damage cells and may be linked to long-term health problems like inflammation, cardiovascular diseases, and possibly even neurodegeneration. These oxidized compounds can also deplete your body's vitamins B and E and may contribute to stomach upset. When in doubt, it is always safer to discard the oil than to risk these complications.

Conclusion: When to Discard Used Fish Oil

The short answer to "how long can you keep used fish oil?" is: not very long. Due to its volatile omega-3s, used fish oil from frying has a limited, unpredictable lifespan, even with proper storage. For fried fish, it's best to reuse the oil only once, if at all, and to use it quickly. Always store it in a sealed, dark container in the refrigerator and strain it carefully. For oil from canned fish, use it within a few days of opening. The most reliable indicators are your own senses. If the oil smells pungent, looks dark or foamy, or tastes bitter, it has gone rancid and should be thrown out immediately. Choosing to be cautious protects both the flavor of your food and your long-term health.

The Guardian: Turn leftover tinned fish oil into tasty dressing

Frequently Asked Questions

Yes, you can reuse oil after frying fish, but only once or twice at most, and preferably only for frying more fish. The oil will absorb the strong fish flavor, making it unsuitable for other foods. Always strain and refrigerate it between uses.

First, let the oil cool completely. Strain it through a fine-mesh sieve with cheesecloth to remove food particles. Pour the strained oil into an airtight, opaque container, label it with the date, and store it in the refrigerator.

Rancid fish oil has a strong, unpleasant, and overly pungent fishy smell, unlike the mild oceanic aroma of fresh fish oil. If you notice a foul odor, discard it immediately.

Yes, oil from canned fish is often high-quality and can be reused as a flavor additive in dressings, pastas, or sauteed dishes. However, it should be treated like a prepared food and used within a few days of opening the can.

Using rancid fish oil can expose your body to harmful free radicals that cause cellular damage and may contribute to inflammation and chronic diseases over time. It can also cause stomach upset and deplete essential vitamins.

Reusing frying oil too many times increases the risk of creating harmful compounds like trans fats and aldehydes. This can lead to oxidative stress and has been linked to issues like cardiovascular diseases and potential neurodegeneration.

To safely dispose of used oil, pour the cooled oil into a non-breakable, sealable container (like a coffee can or the original bottle) and place it in the trash. Never pour it down the drain, as it can cause serious plumbing problems.