Melatonin, the ‘natural’ bedtime pill. But is it really safe?

And given the sheer scale of the data analysed, the resulting findings are difficult to dismiss as mere coincidences. For people taking melatonin long-term, the risk of developing heart failure was significantly higher: around 90 per cent higher than for non-users. And that’s not all. Hospital admissions related to heart problems were much more frequent, and the likelihood of dying from any cause was almost doubled among melatonin users. These figures, at first glance, appear to clearly overturn the mainstream perception of melatonin as a harmless supplement.

Of course, as the authors of the study acknowledge, these are preliminary findings, so these results should be treated with caution. Nevertheless, they are sufficient to raise doubts about this widely available ‘natural’ remedy, both in American chemists and in our own pharmacies.

However, the scientific reality is never as straightforward as it might appear at first glance, and an observational study such as this – even one involving a large sample size – cannot provide any indication of a possible cause-and-effect relationship (between melatonin and heart failure), let alone the pathophysiological mechanisms that might underlie it. The authors themselves, therefore, urge caution: however substantial it may be, this is a preliminary study, which, moreover, has so far only been presented at a major conference but has not yet been published in full, nor has it undergone the rigours of peer review. This study merely indicates a correlation, not conclusive proof of a cause-and-effect relationship. In other words, we cannot say that it is melatonin that causes these effects on the heart. There may be confounding factors that have contributed to this result; for example, people suffering from severe insomnia might be more likely to use melatonin and might already be more susceptible to cardiovascular problems, perhaps because they suffer from anxiety, depression or other mental health conditions that have a negative impact on the heart. In this scenario, melatonin would be more of a signal, a red flag, or a warning sign of an underlying phenomenon (the negative effect of poor mental health on the heart), rather than a cause.

And then there is another unanswered question, perhaps the most important one: is melatonin always harmful, or does it depend on how long we use it for? When taken for short periods and in specific situations (to combat ‘geographical’ jet lag or shift work, or to address temporary bouts of insomnia), its use is generally considered beneficial and relatively safe. The problem arises when the exception becomes the rule – that is, when the use of melatonin supplements continues for months or years, completely outside a doctor’s supervision.

And this is where two often-overlooked aspects come into play. The first is the lack of robust data on the long-term safety of melatonin, particularly with regard to its effects on the cardiovascular system. The second concerns the very nature of supplements: in many countries, they are not regulated as strictly as medicines; this means that dosages, purity and quality can vary considerably from one product to another. What we take every evening before going to bed may not always be exactly the same, with the same dosage and the same bioavailability, and it may even be a product that has not been thoroughly tested.

The key takeaway from this preliminary study – which, however, did have the merit of opening Pandora’s box – is therefore not to demonise melatonin outright, but to put its image as a good, natural and safe ally of sleep into perspective. Melatonin is neither a magic wand nor a universal, ‘one-size-fits-all’ solution. And it may not be entirely risk-free simply because it is marketed as a ‘natural’ remedy. Sleep – real sleep – is the result of a complex balance, comprising healthy habits, regular routines, the right environment and good mental health. Perhaps, therefore, rather than looking for ‘natural’ shortcuts, it is better to ask ourselves a few more questions. Preferably in consultation with your GP.