Flaxseeds and Cancer Warnings: What You Should Know

Written by Rachel Reiff Ellis
Medically Reviewed by Melinda Ratini, MS, DO on March 11, 2025
5 min read

Flaxseeds and hormonal cancer have a complex relationship. Flaxseeds are rich sources of plant-based compounds called lignans. These compounds may act somewhat like the hormone estrogen in your body. Estrogen can be linked to a higher risk of hormone-sensitive cancers, such as breast, endometrial, ovarian, and cervical cancers.

But estrogen can also have some protective effects against some cancers, and this may be the case for flaxseeds, too. 

"Flaxseed is a very concentrated source of dietary fiber, which clearly reduces risk of colorectal cancer," said Karen Collins, RDN, a nutrition adviser to the American Institute for Cancer Research. "And emerging research suggests that higher dietary fiber might play a role in reducing risk of breast cancer."

Flaxseed is also a source of alpha-linolenic acid, a plant form of omega-3 fatty acid. Collins said that evidence is very limited, but it’s possible that this fatty acid could have anti-inflammatory or other effects that contribute to a diet for lower cancer risk.

Understanding how flaxseed consumption may or may not influence cancer risk is challenging because many things are at play, said Collins. 

"Cancers develop over many years, so tightly controlled intervention trials aren’t available," she said. "Studies in large populations that follow people for many years are challenging because of the limited number of people who consume flaxseed. Moreover, it would be difficult to separate the influence of flaxseed from the influence of an overall healthy diet that is likely characteristic of people who regularly include flaxseed."

It may not be surprising, then, that studies on the link between flaxseed and hormonal cancer risk have conflicting results. 

Lignans are in a variety of foods, including seeds, whole grains, legumes, fruits, and vegetables. Flaxseeds are the richest source of lignans. They have 75 to 800 times more than other sources. 

Flaxseed is the small oily seed that comes from the flax plant. You can get it through eating certain foods such as crackers, waffles, and oatmeal, or by adding the seeds, in whole or ground form, to foods you eat. You can also use flaxseed oil.

Your digestive tract can’t absorb lignans directly. Instead, your gut bacteria break them down and convert them to substances called enterolignans that can go into the blood and circulate throughout your body.

The effect of flaxseeds on estrogen and testosterone levels

Scientists sometimes call lignans (and enterolignans) phytoestrogens. This means their chemical structure is similar to estrogen’s chemical structure. 

"This has led to a lot of fear and misunderstandings," Collins said. "Research now shows that estrogen receptors are not all the same. Enterolignans bind more strongly to the type of estrogen receptor called ER-beta that suppresses – not promotes – cancer cell growth."

Enterolignans may also influence the type of estrogen you have most of in your body. The stronger form of estrogen (estradiol) has been linked to a higher risk of breast cancer. Studies in humans are very limited, but some suggest that in postmenopausal women, flaxseed consumption can lead to higher levels of a weaker form of estrogen (enterodiol and enterolactone). This form of estrogen is not linked to breast cancer growth.  

Very small studies also show that flaxseeds may help lower testosterone levels in the body, which can reduce prostate cancer growth. But Collins said these findings haven’t translated into outcomes doctors can use in a clinical setting. 

Ultimately, more research is needed to see if the lignans have an effect on hormonal cancers. 

Studies on flaxseed and cancer prevention are very limited. But some studies on animals suggest that eating flaxseed can lower the risk of hormone-sensitive breast cancer after menopause. Flaxseed and the lignans that scientists extract from it may be able to reduce the chance of breast cancer forming and lower levels of some of the growth factors that promote breast cancer. 

Lab studies on animals also show that lignans may offer protection against other cancers, including ovarian, endometrial, and prostate cancers. But because these studies weren’t done on humans, there isn’t evidence the same effects would happen in people. 

There is "probable evidence" that foods high in fiber – which flaxseed provides – decrease your risk of colorectal cancer, as well as weight gain, overweight, and obesity. These effects on your weight are related to cancer risk, because excess fat is linked to an increased risk of at least 12 types of cancer. 

If you’re living with cancer, talk to your doctor about flaxseed. Eating flaxseed, especially after postmenopausal breast cancer, is likely safe, but more clinical studies are needed, Collins said. 

A few studies following postmenopausal breast cancer survivors link higher dietary or blood levels of lignans with fewer deaths. For people who haven’t gone through menopause, there aren't enough studies to know what the effects may be. 

Some research suggests that eating flaxseeds after a breast cancer diagnosis may slow down how quickly cancer cells divide and reproduce in your body. It may also speed up the rate at which cancer cells die. 

Beyond potential hormonal changes, there are other effects to be aware of when you eat flaxseed. 

In the past, scientists have had concerns that lignans may interact with tamoxifen, a common treatment for hormonal cancers such as breast and endometrial cancers. But studies in mice show that flaxseed and lignans reduce both the formation and the growth of both estrogen-positive and estrogen-negative cancers. 

In fact, flaxseed and flaxseed oil may even help tamoxifen and other breast cancer medications like trastuzumab work better in your body. Again, these studies did not include humans, so more research is needed.

Collins suggests that if you’re thinking of adding flaxseed into your diet, start gradually. 

"Flaxseed is very high in dietary fiber. People who want to start including it may be more comfortable if they start with small amounts – maybe one tablespoon a day – and increase it gradually," she said. 

Consider your timing, too. Have flaxseed at least one hour before or two hours after taking any medication. Otherwise, the high fiber content of flaxseed could decrease how much of your medication your body absorbs.

  • Flaxseeds are oily seeds that contain lignans, plant compounds that act similarly to estrogen. 
  • While some studies show flaxseed's fiber may reduce colorectal cancer risk, and its omega-3 fatty acids could offer anti-inflammatory benefits, the evidence on flaxseed's impact on hormonal cancers is mixed. Some small studies show it may increase cancer risk, especially the risk of hormone-sensitive cancers like breast cancer, while other studies show flaxseeds may have protective effects for some cancers. Lignans may bind to estrogen receptors that suppress cancer growth, and some research shows flaxseed could lower testosterone levels, potentially reducing prostate cancer growth. 
  • More research is needed to understand flaxseed’s full role in cancer prevention.