A study showing potential heart failure problems with taking melatonin is getting a lot of attention. As is often the case with news releases on scientific research, a lot of over reactions and a lot of premature dismissals are taking place. This study is particularly vulnerable to such responses because we don’t know much yet. Right now, the only research publication is a non-peer reviewed short summary for a conference: https://www.ahajournals.org/doi/10.1161/circ.152.suppl_3.4371606.
The study compared two groups from multiple countries. One group got melatonin by prescription; the other group did not. In the USA, melatonin can be bought without a prescription, but it is sold by prescription in some other countries. Subjects were followed for 5 years. The subject number was large (over 65,000 in each group). Heart failure incidence was 4.6% for the melatonin group and 2.7% for the controls. The data showed a high degree of statistical significance (p=0.001, which means the chances are only 1/1000 that the difference between groups was just coincidental). Heart failure related hospitalizations were also mentioned, but I couldn’t make sense of the numbers as reported. Lastly, all-cause mortality was stated to be higher in melatonin users 7.8% vs 4.3%, but no p value was given.
I have a lot of questions about the study that I can’t get answered from just the short summary. For instance, how did the ages of the two groups compare? The summary only says that all subjects were at least 18 years old. If the non-melatonin group had a lot more young adults, that could explain the results. I would also like to know other details about the melatonin group. Maybe the people who were given a prescription tended to have worse health than the people not given the prescription (though all the people in the study were supposed to have insomnia). Another unknown is the types of melatonin doses associated with this study. If the doses varied a lot, it would be interesting to see if most heart failures happened with high doses.
From what is reported so far, it can be stated that even in the melatonin users, over 95% of the people did not have a heart failure incident. This may have to do with the ages of the people in the study. If a lot of the people were under 50, not a lot of heart failures would be expected. Maybe, the study data will be eventually analyzed by age subsets. Possibly, the older people in the study will show a greater heart failure incidence compared to controls.
So, to me, the jury is still out as to what this study means. I will say that the study raises caution flags about long term, high dose melatonin use. Still, until I see more about the study, I can’t say how much concern should be raised. Nonetheless. I have always had theoretical concerns about long term, high dose use of melatonin. This compound is called a nutrition supplement because nuts contain some. Even so, melatonin is more of a hormone than a nutritional supplement.
Our body only makes about 0.05 to 0.10 mg melatonin at night. In contrast, many people take 5 mg or more a day. Even if such oral doses only have about 15% absorption, those doses can still dump relatively high amounts into the blood. Despite this, some people in the supplement industry declare that melatonin has been shown to be safe. In contrast, studies besides this recent one have popped up that raise safety questions. So, at the moment, I retain some reluctance to endorse long term, high dose melatonin use.