Melatonin is an indolamine hormone which is secreted from the human pineal gland during night-time acting as physiological regulator. In many countries, dietary supplements containing synthetically produced melatonin are available. Melatonin is being promoted as a treatment of a range of conditions, including virtually all types of cancer.
One website, for instance, states that the anti-cancer benefits of melatonin aren’t just indirect; this miracle molecule is also classified as a directly cytotoxic hormone and anti-cancer agent. Studies have referred to melatonin as a “full-service anti-cancer agent” due to its ability to inhibit the initiation of cell mutation and cancer growth, and to halt the progression and metastasis of cancer cell colonies.
Such statements sound far too good to be true. So, let’s have a look and find out what the evidence tells us. Test-tube experiments suggest that melatonin has anti-cancer effects. Its actions include the advancement of apoptosis, the arrest of the cell cycle, inhibition of metastasis, and antioxidant activity.
A review of 21 clinical trials of melatonin for cancer found positive effects for complete response, partial response, and stable disease. In trials combining melatonin with chemotherapy, adjuvant melatonin therapy decreased 1-year mortality and improved outcomes of complete response, partial response, and stable disease. In these studies, melatonin also significantly reduced asthenia, leukopenia, nausea and vomiting, hypotension, and thrombocytopenia. The authors concluded that melatonin may benefit cancer patients who are also receiving chemotherapy, radiotherapy, supportive therapy, or palliative therapy by improving survival and ameliorating the side effects of chemotherapy.
A further systematic review of RCTs of melatonin in solid tumour cancer patients evaluated its effect on one-year survival. Ten trials were included of melatonin as either sole treatment or as adjunct treatment. Melatonin reduced the risk of death at 1 year. Effects were consistent across melatonin dose, and type of cancer. No severe adverse events were reported.
A 2012 systematic review confirmed these findings by concluding that Melatonin as an adjuvant therapy for cancer led to substantial improvements in tumor remission, 1-year survival, and alleviation of radiochemotherapy-related side effects.
Finally, a 2020 review concluded that melatonin in combination with anticancer agents may improve the efficacy of routine medicine and survival rate of patients with cancer.  Apart from its direct anticancer potential, melatonin also seems to reduce chemotherapy toxicity, while improving its therapeutic efficacy.
So, is this evidence compelling? While all this does indeed sound encouraging, it is necessary to mention several important caveats:
- The primary studies of melatonin suffer from several methodological shortcomings.
- Their vast majority originate from one single research group.
- In recent years, there have been no further clinical studies trying to replicate the initial findings.
This means that definitive trials are still missing, and it would seem wise to interpret the existing evidence with great caution.
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