Despite its antioxidant effects and role in regulating sleep cycles, melatonin, popularly known as the "sleep hormone", can worsen the inflammation of the intestine and impair the action of gut microbiota, warned a study.

The community of bacteria and other microbes in the gut is important to health, helping control digestion and benefiting the immune system. An imbalance of unhealthy and healthy microbes in the intestines may contribute to weight gain, high blood sugar, above-normal cholesterol and other disorders.

Melatonin is often taken as a so-called food supplement without a doctor's prescription by people with sleep problems.

"It's generally thought to be harmless. After all, it's a hormone and can help regulate sleep. However, our study shows that people should be careful about taking hormone supplements and that the ingestion of melatonin supplements can have adverse effects on health," said Cristina Ribeiro de Barros Cardoso, Professor of immunology and neuro immunoendocrinology at the University of Sao Paulo, Brazil.

The study, published in the journal Microorganisms, involved an experiment in which colitis was induced in an experimental model, and they were treated with melatonin. Their condition became worse instead of improving.

"It's important to stress that no human patients were involved in the study. The model's bowel inflammation became much, much worse," Cardoso said.

"We then began trying to understand why. We found that melatonin had a positive effect on the disease if the effect on gut microbiota was ignored and the experimental model was treated with wide-spectrum antibiotics to eliminate all the bacteria."

The negative effect of melatonin, therefore, depends on the bacteria that live in the intestine and are also associated with inflammatory diseases of the region.

Certain features of gut microbiota increase inflammation and dysregulate the immune system in response to treatment with melatonin, damaging the digestive system.

"We should take great care with medications, hormone supplements or hormones offered as food supplements. You buy a 'food supplement' in a pharmacy and think it's not a drug, it won't alter anything in your body, it will only do good because after all it's sold as a food supplement, but it's not really that at all. It's a hormone, and regulation of the interaction between all hormones and the immune system is very delicate," Cardoso said.