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★☆☆Contested — the evidence disagrees with itself, or the finding carries a caution. A caution or conflict always shows one star, however strong the sources are.
Nobody has confirmed the proposed ways melatonin might harm the heart
Two papers suggest long-term melatonin might keep prolactin raised or blunt the night-time dip in blood pressure. Neither has been shown in people taking it.
What the study found
The proposed routes from melatonin to heart failure are unconfirmed
Two papers offer routes, among them that long-term melatonin keeps the hormone prolactin raised, or that the body stops responding to it and loses the night-time dip in blood pressure. Neither has been shown in people taking it. They are hypotheses, not findings.
The caveat · Built on one unreviewed report
Both papers were written to explain a single conference report that had not been peer-reviewed. The letter proposing the blood-pressure route also says untreated insomnia could explain the same result.