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What Is the Connection between Melatonin and Breast Cancer?

By C. K. Lanz
Updated Jan 23, 2024
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Several studies suggest a connection between melatonin and breast cancer characterized by a reduction in tumor size when the supplement is added to treatment. Breast cancer patients often have lower levels of melatonin compared to healthy individuals. A low level of melatonin was found to stimulate the growth of some types of breast cancer cells in research studies. Melatonin and breast cancer research shows that raising a patient’s melatonin levels may slow tumor growth, augment the effects of certain chemotherapy drugs, and stop platelet levels from lowering in the blood.

Melatonin is a hormone that the pineal gland secretes in order to maintain the body’s circadian rhythm, or internal clock. Light causes the production of melatonin to drop, so exposure to too little or too much light can disrupt the body’s melatonin cycle. This hormone also affects the timing and release of reproductive hormones in women.

Much melatonin and breast cancer research focuses on linoleic acid, a dietary fat that can encourage breast cancer growth. Tumors grow more quickly by absorbing more linoleic acid when the body is exposed to light and melatonin levels drop. Keeping lights on at night suppresses natural melatonin cycles and allows tumors a longer growth period each day.

Melatonin interacts with linoleic acid, slowing its production down and interrupting tumor growth. This hormone is a type of anticancer signal to human breast cancers, and approximately 90 percent of these cancers have receptors for it. This is the crux of the link between melatonin and breast cancer: The hormone helps put tumors to sleep so that they do not grow as quickly. Studies suggest that melatonin can slow the rate of breast cancer growth by as much as 70 percent.

Other studies suggest an additional connection between melatonin and breast cancer. The hormone may increase the effects of some chemotherapy drugs used to treat breast cancer. In one study, women with breast cancer that was not responding well to tamoxifen added melatonin, resulting in 28 percent of participants experiencing tumor shrinkage. Another study showed that patients given melatonin a week before chemotherapy maintained healthy platelet levels and prevented thrombocytopenia, a complication that can lead to prolonged bleeding.

There are several available forms of melatonin, such as tablets and capsules, lozenges that dissolve under the tongue, and a cream. As there is no recommended dose for this supplement, most individuals begin with a very lose dose close to the amount normally produced daily by the body: less than 0.3 milligrams. If this dosage is not effective, a doctor can advise a patient who wants to increase his or her daily intake.

Melatonin is used to treat a variety of conditions, including insomnia, menopause, and attention deficit hyperactivity disorder. Its primary use is to alleviate insomnia and jet lag. Less common uses are as a sunscreen and a treatment for irritable bowel syndrome and epilepsy.

The main side effect of melatonin is sleepiness, but some people may experience vivid dreams or nightmares. A higher dose can trigger irritability and anxiety in some people as well as gastrointestinal problems. Some research suggests that it can worsen depression and interfere with fertility. As with any supplement not regulated by the United States Food and Drug Administration or a similar agency in their home country, individuals considering melatonin should consult with their doctors prior to starting a regimen.

WiseGeek is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
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