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If you find it challenging to fall asleep and stay asleep, you’re not alone. Melatonin is one of the most ubiquitous supplements in the U.S., with various forms of it available at most grocery stores and pharmacies. In fact, research shows that between 1999 and 2018, melatonin use in the U.S. quadrupled1. (It’s more strictly regulated in other countries.)

That said, research shows that we don’t really know enough about the safety of supplementing with melatonin, especially using it every night—sometimes for years on end. As the authors of one study2 point out: “Although melatonin is generally regarded as safe, adverse effects have been reported, and data on long-term use and high-dose use are scarce.” 

Some sleep experts worry that using melatonin regularly may reduce your body’s natural productivity of this important sleep hormone.

As Seema Bonney, M.D., the founder and medical director of the Anti-Aging & Longevity Center of Philadelphia, previously told mindbodygreen: “It’s important to remember that melatonin is a hormone, and using any hormone regularly can down-regulate your own production of that hormone.” Ashley Jordan Ferira, Ph.D., RDN, mbg’s vice president of scientific affairs, added: “I have not seen good data to show that high doses of melatonin will not impact your endogenous, natural production of melatonin.” 

Some research has also shown that taking melatonin leads to impaired glucose tolerance, which can harm your blood sugar health in the long term. In one study3 published in the journal Sleep, researchers gave 21 healthy women 5 milligrams of melatonin or a placebo in the morning and evening and then monitored their glucose tolerance for 3 hours after each dose. They found that those given melatonin had higher blood sugar levels.

There’s also significant cross talk between the hormone melatonin and sex hormones like estrogen and testosterone, leading some experts to worry about melatonin’s possible long-term side effects on reproductive health.