Life Extension Magazine®

Woman self checking for breast cancer and Petri dish testing melatonin

Melatonin: A Promising Protector Against Breast Cancer

Researchers have found that melatonin can protect against breast cancer risk factors, impede the growth/spread of tumor cells, and boost the effectiveness of certain cancer treatments.

Scientifically reviewed by: Dr. Shaylind Benson, ND, in August 2023. Written by: Juan Pablo Bustos, MD, MSC..

Melatonin is best known for helping to induce sleep.

Studies show melatonin may also play a role in the fight against breast cancer.

Intriguing research shows that melatonin has an important role in impeding the growth and spread of breast cancer cells.1-6

Clinical trials demonstrate how melatonin can boost effectiveness of existing cancer treatments while reducing their side effects.7,8

Adding melatonin to the cancer drug tamoxifen in a Petri dish made the drug 100 times more potent as an inhibitor of breast cancer cell growth.9

Melatonin can also help combat lifestyle and environmental breast cancer risk factors.

What you need to know

  • Melatonin, a hormone known for inducing a good night’s sleep, has been used for many years in the treatment of symptoms associated with breast cancer.
  • New research reveals melatonin’s anticancer properties, and lab studies show that it can slow the growth and spread of breast cancer.
  • The addition of melatonin to conventional breast cancer therapies has been shown to boost effectiveness and reduce side effects.
  • Studies show that melatonin can help protect against breast cancer risk factors, including certain environmental exposures, obesity, and being exposed to light at night.

Melatonin’s Anti-Cancer Properties

Melatonin’s Anti-Cancer Properties  

For years, melatonin’s medical use has been largely limited to treating the symptoms associated with breast cancer—such as improving sleep10 and improving depression and anxiety.11

New research reveals that melatonin has anti-cancer properties.6,12 Most notably, lab and animal studies have shown that it can slow or stop the growth and spread of breast cancer.1-6,13

This is likely due to melatonin’s impact on estrogen.

About 70% of breast cancer tumors are growth-sensitive to estrogen,14 meaning the hormone fuels the cancer’s growth. Many breast cancer therapies are aimed at either decreasing circulating estrogen or reducing tumor sensitivity to the hormone.

Melatonin has natural anti-estrogen effects. It limits the amount of the enzyme that changes estrogen into a more active form, while also increasing the enzyme that keeps circulating estrogen in an inactive state.15-23

Studies done on human breast cancer cells have shown that melatonin reduces the sensitivity of estrogen receptors on the surface of breast cancer cells, which reduces the cells’ growth response.24,25

Melatonin Improves Breast Cancer Therapies

The impact of melatonin’s anti-estrogen properties is most obvious in its ability to boost the effectiveness of tamoxifen, a commonly used adjuvant breast cancer drug.

Tamoxifen works by blocking estrogen receptor sites, which helps slow the growth and reproduction of many breast cancer cells.

In a study of breast cancer cells, adding melatonin to tamoxifen made the drug 100 times more potent in inhibiting the growth of those cells.9 This means that it may be possible to use less of the drug to achieve similar effects—saving money while reducing harmful side effects.

In addition to boosting the effectiveness of certain cancer therapies, melatonin can also reduce their side effects.7,8

For example, in an animal study, it reduced the secondary liver damage that may be caused by the drug letrozole (another hormone-based treatment for breast cancer).26

Another side effect of breast cancer treatment is an increased risk for osteoporosis.27 Melatonin has been shown to prevent the loss of bone tissue by stimulating the production of new bone and decreasing the reabsorption of calcium from the bone, a process that diminishes bone density leading to osteoporosis and increased risk for fractures.28-31

Due to its properties as a free-radical scavenger, melatonin may also counter the calcium-depleting effect of free radicals on bone, leading to a more stable bone matrix.32

In one randomized, controlled clinical trial, women who received 1 or 3 mg a day of melatonin for one year showed improvement on bone density markers when compared to women who had received a placebo. Women who received the higher doses of melatonin demonstrated the most marked improvements in bone density.33

Because of the beneficial effects of melatonin on preserving bone density and the prevention of osteoporosis, it is reasonable to consider using melatonin in conjunction with common breast cancer therapies.

And when tested in combination with six different chemotherapy drugs (for a variety of different types of cancer), using melatonin (20 mg/day) in addition to the drugs resulted in:

• A significantly higher survival rate after one year, and

• Significantly reduced the toxicity of the drugs.7

Furthermore, melatonin has been found to make cells from breast tumors more sensitive to the effects of chemotherapy drugs while also preventing resistance to their anti-cancer effects.34

Boosting Radiation Effectiveness

Melatonin’s effects are equally impressive when used in conjunction with radiation therapy.

In one study, when cancer patients undergoing chemotherapy and radiation took melatonin (20 mg every night) and a melatonin gargle during their seven weeks of treatment, they experienced fewer side effects and were able to tolerate treatment more consistently, compared to a placebo group.8

Research done on tumor cells has shown that exposure to melatonin before treatment increased the effectiveness of radiation therapy by decreasing cell proliferation.35

Melatonin can also help prevent the skin irritation caused by radiation therapy, as demonstrated by results from a clinical trial that used a melatonin-based cream during and after radiation sessions. Women who used the melatonin cream had significantly less skin irritation after radiation treatment compared to a placebo group.36

Together, these studies show that using melatonin in addition to breast cancer therapies can enhance their effectiveness while reducing their toxicity.

Melatonin Reduces Environmental Risk Factors

Melatonin Reduces Environmental Risk Factors  

As helpful as it is to be able to boost the effectiveness of breast cancer treatments, the ultimate goal is to avoid developing breast cancer to begin with.

New research has demonstrated a number of ways melatonin can protect cells—or diminish the cell damage—caused by a variety of environmental factors that play a major role in disease development.37

  • Tobacco. Both smokers and those exposed to secondhand smoke face an increased risk for developing breast cancer.38 Studies in rodents show that melatonin reduces the oxidative damage caused to cells by exposure to cigarette smoke.39 By reducing the damage to the cell induced by cigarette smoke, many precancerous lesions were either improved or avoided altogether.

  • Acrylamide. This environmental contaminant is released when certain foods are cooked at high temperatures.40 Acrylamide can disrupt the normal function of cells by causing oxidative damage and corrupting DNA. Research done in rats showed that melatonin reduced the oxidative damage caused by exposure to acrylamide while also diminishing damage to DNA.41

  • Polycyclic Aromatic Hydrocarbons. This group of chemicals is generated by incomplete combustion of fuels such as wood, coal, and gas. Major sources of exposure include residential heating, motor vehicle exhaust, and fossil fuel-intensive industrial processes, including refineries.42 Animal research shows that these compounds cause cellular changes in breast tissue that lead to the development of tumors.43 When animals were exposed to a form of polycyclic aromatic hydrocarbons, treatment with melatonin significantly reduced the number and size of tumors.44

  • Cadmium. Cadmium is a ubiquitous environmental contaminant present in the food chain that plays a role in the development of breast cancer.45,46 A survey conducted in the U.S. revealed a cadmium exposure prevalence of over 93% of the population.47 In animal studies, melatonin has been shown to protect against oxidative stress caused by cadmium,48,49 while also countering the negative effects of cadmium on breast tissue.50

  • Light. Exposure to light at night is a commonly overlooked risk factor for breast cancer.5 Nighttime exposure to light disrupts natural melatonin secretion by the pineal gland,5,51 raising the risk for developing the disease.52-56 Supplementing with melatonin has been shown to counter some of the negative effects of exposure to light at night.57

Obesity and Breast Cancer

Obesity is a major risk factor for the development of breast cancer, especially among postmenopausal women.58,59 In one controlled clinical trial, melatonin showed promising anti-obesity effects, which is of great scientific and research interest.60,61 Melatonin has also shown the ability to reduce some harmful, breast-cancer-inducing effects of obesity.

In one randomized, double-blind, placebo-controlled trial, postmenopausal women who took 1 or 3 mg of melatonin nightly for a year had significant decreases in fat mass and increases in muscle mass, compared to the placebo group.60

Research shows beneficial effects of melatonin supplementation in obesity as well as for its related complications.61

In postmenopausal women, obesity promotes the overexpression of aromatase, an enzyme that stimulates the production of estrogen. In breast tumors, estrogen can reach concentrations up to 10-fold higher than in blood.62 Such a high local concentration of the hormone causes tumor initiation and progression.63

In addition to its anti-estrogenic activity, melatonin has been shown to inhibit the activity and expression of aromatase.21

Exposure to Light at Night May Increase the Risk for Breast Cancer
Exposure to Light at Night May Increase the Risk for Breast Cancer

Melatonin plays a key role in the regulation of the body’s sleep/wake cycle and helps establish circadian rhythm, the 24-hour schedule of biological processes that cells and systems follow to maintain health and carry out their functions.

In order to maintain a proper circadian rhythm, the body depends on a 24-hour cycle of alternating patterns of light and darkness. Altering these patterns of light and darkness may have an effect on metabolism and cell function. Evidence from several studies shows that exposure to sources of light at night leads to the interruption of circadian rhythm and increases the risk of cancer.

This is especially significant in women who are already predisposed to cancer, such as those with a family history, or who have abnormal BRCA1, or BRCA2 genes that increase the risk of developing breast and ovarian cancer.

Research shows that the light emitted by digital screens (smartphones, tablets, laptops), along with the internet and social networking related activities, could disturb the normal pattern of sleep in humans and have a negative effect on normal melatonin release.

Exposure to light at night has also been associated with other metabolic, psychiatric and behavioral disorders. The World Health Organization has classified night-shift work and exposure to light at night as a “probable carcinogen to humans.” Limiting the use of digital screens at night, or using filter applications that diminish light may help reduce the negative effects of exposure to light at night.64


Melatonin offers promise for the prevention and management of breast cancer.

In addition to slowing growth and spread of breast cancer cells, melatonin may boost the effectiveness of cancer treatment, while also reducing harmful effects.

Studies show that melatonin helps protect against breast cancer risk factors, including certain environmental exposures, obesity, and being exposed to light at night.

Scientists are considering incorporating melatonin as an adjuvant approach in the treatment of breast cancer.

If you have any questions on the scientific content of this article, please call a Life Extension® Wellness Specialist at 1-866-864-3027.


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