Short answer: research so far does not report that melatonin causes cancer, but the studies covered here do not directly test that question, and long-term use has not been fully studied.
What the lab research on melatonin and cancer shows
Lab tests use cells in a dish or animals, not people. Many of the anti-cancer findings for melatonin come from lab cells and animals. The 2018 Molecules review noted ten clinical trials of melatonin in breast cancer listed at ClinicalTrials.gov. Five focused on relieving symptoms. The other five tested melatonin in women already diagnosed, and their results were still being analyzed. A 2021 study described by Healthline looked at tissue from triple-negative breast cancer and found melatonin may suppress a cell signaling pathway involved in that cancer’s growth. A 2025 study in rats suggested melatonin may help reduce the growth of glioblastoma tumors.
The clinical-trial picture is still developing. The 2018 review concluded that clinical trials testing melatonin alongside standard treatments were encouraged. Cancer research is moving on many fronts; one same-site report covers a personalized cancer vaccine phase 3 melanoma result, which is a separate area from melatonin.
Does melatonin cause cancer, or raise estrogen?
Healthline states that melatonin hasn’t been shown to lower estrogen levels, and that research suggests it can modulate estrogen responsiveness. It also says melatonin may alter hormone levels if taken long-term, and that long-term use hasn’t been fully studied. The 2018 review describes lab experiments in which melatonin reduced signals from the estrogen receptor in breast cancer cells and slowed enzymes that make active estrogen. Those are cell experiments, not a measured rise in estrogen in people.
If you take hormone therapy, bring that to your clinician. The 2018 review lists combination hormone therapy among factors with adequate evidence of increased breast cancer risk.
What are the best ways to lower my cancer risk?
Research points to a few factors you can influence. A Johns Hopkins article on sleep and cancer says, as of October 2026, that long stretches of shift work may increase cancer risk, because disruptions in the body’s biological clock may raise the odds of breast, colon, ovarian and prostate cancers. A 2009 Cancer Letters review found that night shift work has been consistently linked with breast cancer and, more recently, endometrial cancer.
- Shift work and light at night. The 2018 review cites an estimated 16% increase in breast cancer risk for every 10 years of shift work.
- Obesity and alcohol. The 2018 review, citing the National Cancer Institute’s 2017 summary, lists both among factors with adequate evidence of increased breast cancer risk.
- Sleep habits. The NHS sleep guidance notes that alcohol and nicotine can disrupt sleep, so it advises avoiding them one to two hours before bed.
Can I take melatonin if I’ve had breast cancer or I’m in treatment?
Healthline says melatonin is considered safe for most people with breast cancer, but it can interact with several medications. Those include medicines for diabetes, hypertension, seizures, heart conditions, autoimmune conditions and mental health conditions. Healthline also reports no noted interactions with chemo drugs, and says there is not enough evidence to say whether melatonin can prevent breast cancer recurrence. Ask your oncology team before you start, and share every medication and supplement you take.
On chemotherapy, a PubMed-indexed review on melatonin and genomic stability says melatonin has shown some synergistic properties with chemotherapy and radiation, and that it attenuates several side effects of both. Healthline reports a 2021 research review that suggests melatonin may improve the effectiveness of cancer treatment, and other studies that suggest it may help with chemo-related fatigue. Healthline adds that more research is needed.
Studies in this area cover breast, prostate and brain (glioblastoma) tumors. For lymphoma, or for the question of taking melatonin during chemotherapy, bring the question to your oncology team.
Is 10 mg of melatonin too much every night, or for years?
The studies covered here do not say whether 10 mg or 20 mg nightly is too much for you. Ask your pharmacist or oncology team which amount, if any, fits your situation and how long it should be taken.
Healthline notes that the long-term use of melatonin hasn’t been fully studied, and that melatonin may alter hormone levels if taken long-term. That’s the reason to review any nightly use with your clinician, especially if you’re thinking about taking it for years.
What are the side effects, and who should be careful?
| Concern | What the research or guidance says | Who should be careful |
|---|---|---|
| Sleepiness, dizziness, nausea | Healthline lists these as known effects of melatonin for people with breast cancer. | People with breast cancer, per Healthline. |
| Medication interactions | Healthline says it can interact with several prescription medications and recommends asking a doctor first. | People taking medicines for diabetes, blood pressure, seizures, heart, autoimmune or mental health conditions. |
| Hormone changes | Healthline says melatonin may alter hormone levels if taken long-term. | People taking it long-term. |
| Long-term safety | Healthline says long-term use hasn’t been fully studied. | Anyone planning long-term nightly use. |
Cleveland Clinic’s sleep expert cautions against using melatonin unless a person has had a full evaluation of their sleep difficulties, and notes that it works best for people with a delayed circadian rhythm, sometimes called night owls. For the amount that counts as too much, ask your pharmacist or doctor before you take any.
What is melatonin used for?
Melatonin is best known as a sleep aid. Cleveland Clinic describes it as an over-the-counter supplement that can partially help manage the body’s sleep-wake cycle. Research into melatonin also covers its role in circadian rhythms, seasonal behavior, and its antioxidant and immune effects. A PubMed review on melatonin and apoptosis describes melatonin as having tumor-inhibiting properties in experimental models.
Why don’t doctors like melatonin?
The caution is about evaluation and long-term data, not a finding that melatonin is harmful. Cleveland Clinic’s advice is to get a full evaluation first, because other causes can keep people awake. Healthline reports that there are no official recommendations about melatonin from any major health agencies. Long-term use also hasn’t been fully studied.
What if I take melatonin and still can’t sleep?
Cleveland Clinic says most people fall asleep within 10 to 20 minutes, and that it’s fine if it takes longer. The Cleveland Clinic sleep tips advise a consistent schedule, less screen time, and lower stress before bed. The NHS advises that if you’re still awake after around 20 minutes, get up, do something relaxing, and go back to bed when you feel sleepy. If you often have trouble falling asleep, Cleveland Clinic recommends seeing a sleep medicine specialist.
What are my sleep options during cancer, side by side?
Johns Hopkins Kimmel Cancer Center expert Kathryn Ruble says that in her team’s research, nearly one in four survivors of childhood cancer had difficulty falling asleep and staying asleep. Sleep is a real problem during treatment, and the options differ.
| Option | What the research or guidance says | Who it may suit | Cost and limits |
|---|---|---|---|
| Melatonin (over-the-counter supplement) | Partly helps manage the sleep-wake cycle, per Cleveland Clinic; cancer research so far is lab, animal and early work, with more research needed. | Often discussed for night owls with a delayed circadian rhythm, per Cleveland Clinic. | Prices vary; check the label or your pharmacy. Interactions and long-term use need a clinician. |
| Melatonin in breast cancer clinical trials | Ten breast cancer trials were listed at ClinicalTrials.gov at the time of the 2018 review. Five focused on relieving symptoms; the other five tested melatonin in women already diagnosed, with results still being analyzed. | People who qualify for a specific trial. | Ask your oncology team whether a trial is open to you. |
| CBT-I (cognitive behavioral therapy for insomnia) | Cleveland Clinic calls it a first-line treatment and says it tends to have much better long-term outcomes than a sleep aid or sleep medication. | People who have difficulty falling asleep at least three times a week and have daytime impairment, per Cleveland Clinic. | Progress often takes four to six sessions. Ask your insurer about coverage. |
| Sleep aid or sleep medication | Cleveland Clinic says CBT-I tends to have much better long-term outcomes than a sleep aid or sleep medication. | The studies covered here do not compare specific sleeping pills for people with cancer. | Ask your clinician about any prescription option first. |
| Magnesium | Cleveland Clinic says it may help some people sleep better, but there’s still not enough research to know how well it works. | Not a first step for ongoing sleep problems, per Cleveland Clinic. | Check the label; Cleveland Clinic notes the FDA doesn’t regulate supplements like magnesium. |
| Sleep routine and habits | NHS and Cleveland Clinic both recommend a consistent schedule, a cool, dark, quiet room, and limiting caffeine and alcohol. | Anyone starting with the basics. | No product cost. |
Research does not compare specific sleeping pills for people with cancer. Cleveland Clinic’s first-line recommendation for chronic insomnia is CBT-I, and that’s the option to ask your team about first. Our melatonin and alcohol guide covers how drinking changes the picture.
How do I check a supplement’s cancer claim?
In a November 5, 2020 FDA warning letter to Quten Research Institute about CoQ10 and turmeric products (not melatonin or cancer claims), the FDA said website claims of disease treatment or prevention can make a product an unapproved new drug. Before you buy any supplement, read the label and the seller’s claims, and bring them to your oncology team.
Questions to bring to your oncology team
- Is melatonin appropriate with my cancer type, treatment and current medications?
- Has my sleep been evaluated for other causes?
- Is CBT-I available to me, and how would I access it?
- If I take melatonin, what amount and for how long, and how will you monitor me?
- Does my hormone therapy or other treatment change the picture?
By HealthDataConsortium.org Research Team
This article is for general information purposes only and does not constitute medical advice. Consult your doctor or qualified healthcare provider before making changes to your health routine.
