If you’ve ever been told to stop taking fish oil during cancer treatment or before surgery, you’re not alone. Many doctors and surgeons have long warned that fish oil “thins the blood” and could increase bleeding.

But recent research tells a different story. Several large studies and reviews now show that fish oil is safe for most people—even during cancer treatment—and may actually offer some important benefits, like lowering inflammation and helping the body recover from treatment or surgery.

Let’s take a closer look at what science now says about fish oil and cancer care.

Why People Take Fish Oil

Fish oil supplements are rich in omega-3 fatty acids—mainly EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid). These healthy fats come from oily fish like salmon, anchovies, and sardines, and can also be obtained from chia seeds, walnuts or flax seeds. Research is clear about the benefits of omega-3 fats, and the benefits include:

• Reducing chronic inflammation
• Supporting heart and brain health
• Maintaining muscle and joint function
• Improving mental health.

For people with cancer, research suggests omega-3s may also help reduce fatigue, protect muscle mass, and improve appetite—all common challenges during treatment.

Where the “Blood-Thinning” Concern Came From

Years ago, early lab experiments found that omega-3s could slightly reduce how sticky blood platelets are. That’s why doctors worried fish oil might make you bleed more, especially if you’re already taking a blood thinner or preparing for surgery.

However, lab studies don’t always match what happens in real patients. Over the past decade, researchers have taken a much closer look—and what they’ve found is reassuring.

Large Studies Show No Increase in Bleeding

One of the biggest studies on this topic is called the OPERA trial, which included more than 1,500 people who took high doses of fish oil before heart surgery. The results? People who took fish oil did not bleed more than those who took a placebo. In fact, they needed fewer blood transfusions during surgery.

Other reviews have supported these findings. When researchers have pooled data from many clinical trials, they’ve found no clear link between fish oil and an increased risk of bleeding, even when people were also taking common blood thinners.

So, the belief that fish oil “thins the blood too much” appears to be more myth than fact.

What About Cancer Patients?

That same question—does fish oil increase risk during treatment?—has been explored specifically in people with cancer.

A 2025 review in the Journal of Nutritional Oncology looked at dozens of studies of fish oil use during chemotherapy, radiation, and surgery. It found that omega-3 supplements (up to 3 grams of EPA and DHA per day) were generally safe, with only minor side effects like mild stomach upset in some people.

The authors also found no evidence of increased bleeding in patients, even when undergoing surgery or using standard medications. In fact, omega-3s helped lower inflammation, supported immune health, and might improve how some people tolerate treatment.

Fish Oil, Inflammation, and Recovery

Chronic inflammation plays a key role in cancer development and progression—and it can also make recovery harder. High inflammation can worsen fatigue, slow wound healing, and even make it harder to maintain muscle mass.

That’s where omega-3s stand out. They help the body create natural compounds which calm inflammation safely—without shutting down the immune system.

In studies with cancer patients, fish oil supplements have helped to:

• Lower inflammatory markers like IL-6 and TNF-α;
• Preserve lean muscle and reduce weight loss caused by cancer (cachexia);
• Improve appetite and overall energy levels.

So instead of being harmful, fish oil may actually support healing and resilience during treatment.

Fish Oil May Even Affect Tumour Growth in Prostate Cancer

A recent study gives even more reason for optimism. The CAPFISH-3 trial, published in 2024 in the Journal of Clinical Oncology, looked at men with early-stage prostate cancer who were being closely monitored (“active surveillance”).

Half the group ate a high omega-3, low omega-6 diet and took fish oil supplements for one year. The others followed their usual diets. At the end of the study, the men taking fish oil had a 15% drop in tumor cell growth markers, while the control group saw a 24% increase. Importantly, there were no major side effects or bleeding issues in the fish oil group.

This doesn’t mean fish oil cures cancer—but it suggests that it may influence tumour biology in positive ways, especially when combined with a healthy diet.

Is Fish Oil Safe Before Surgery?

Traditionally, surgeons have asked patients to stop taking fish oil a week or two before an operation. The concern, again, was about bleeding.

But new evidence shows that’s often unnecessary. The OPERA trial and other surgical studies found no extra bleeding risk when fish oil was continued through surgery. Some patients even seemed to recover better, possibly because of reduced inflammation and better blood vessel health.

Practical Tips for People in Cancer Treatment

1. Stick to Safe Dosages
Most research supports using 1–3 grams per day of combined EPA and DHA as both safe and effective. Higher amounts are sometimes prescribed for specific goals—such as fighting inflammation or muscle loss—but should be medically supervised.

2. Choose a High-Quality Product
Look for supplements that are third-party tested for purity and potency, made from wild Alaskan fish oil or other certified sources and free from artificial fillers and contaminants.

Liquid forms can be easier to absorb, while capsules offer convenience and less “fishy” taste.

3. Include Natural Sources
If you’d rather get omega-3s from food, try adding:
-Salmon, mackerel, or sardines twice a week or
-Chia seeds, flaxseeds, or walnuts (plant-based sources, though less potent).

Food-based omega-3s can complement supplements or replace them entirely for some people.

Bottom Line: Fish Oil Is Safe—and Possibly Helpful

The old idea that fish oil “thins the blood” too much for surgery or cancer patients simply doesn’t hold up to modern research. Across dozens of well-designed studies, taking omega-3 supplements appears safe for most people on cancer treatment, and may even help reduce inflammation, preserve muscle, and support recovery.

Rather than stopping fish oil out of caution, the conversation is shifting toward how to use it wisely—in the right doses, for the right patients, and as part of a balanced approach to care.