Melatonin has not been clearly proven to cause gynecomastia on its own, but there is a plausible hormonal pathway and at least one documented case where it contributed to breast tissue growth in a child. The risk appears to be very low for most adults taking standard doses, though certain factors like young age, high doses, and combination with other medications may increase susceptibility.
How Melatonin Affects Hormones Linked to Breast Growth
Gynecomastia happens when the balance between estrogen and testosterone shifts enough to stimulate breast gland tissue. Melatonin can influence this balance through two main pathways: prolactin and sex hormones.
Melatonin stimulates prolactin release. A study in women found that a 1 mg oral dose of melatonin during the daytime triggered prolactin secretion similar to what normally occurs at night. Women who already had mildly elevated prolactin levels appeared unusually sensitive to this effect. Prolactin, when chronically elevated, is a well-known driver of breast tissue growth in both sexes. The National Center for Complementary and Integrative Health acknowledges this concern directly, noting that melatonin supplements could cause overproduction of prolactin, though the evidence remains incomplete.
Melatonin also interacts with sex hormones, but the picture is complicated. Animal and cell studies show melatonin can inhibit aromatase, the enzyme that converts testosterone into estrogen, which would theoretically protect against gynecomastia. At the same time, melatonin has been shown to suppress testosterone production in some animal models. In Syrian hamsters, testicular melatonin injections significantly decreased testosterone levels. The net effect in humans taking supplement doses isn’t well characterized, but in children and adolescents, melatonin levels are negatively correlated with testosterone, and elevated melatonin has been linked to delayed puberty.
The One Documented Case
Only one published case report directly links melatonin use to breast tissue development. A 10-year-old boy was prescribed both risperidone (0.5 mg/day) and melatonin for sleep and behavioral issues. His melatonin dose was gradually increased to 4 mg/day over several weeks. After one month at that dose, he developed painful breast budding on his left side, measuring about 1.5 by 2.0 centimeters.
His prolactin was slightly elevated, while both estrogen and testosterone were below detectable levels. This is important because it suggests the breast growth was driven by prolactin rather than a shift toward estrogen. Both risperidone and melatonin independently raise prolactin, so the combination likely amplified the effect. The breast budding disappeared within two days of stopping melatonin.
The authors noted this was the first reported case of breast budding associated with this drug combination, and that most studies on melatonin’s safety have focused on adults over short periods. A prepubescent child with very low baseline sex hormones is far more vulnerable to even mild prolactin elevation than an adult male would be.
Why the Risk Appears Low for Most Adults
Large reviews of long-term melatonin use in adults have found no significant difference between melatonin and placebo in terms of side effects, and the side effects that do occur tend to resolve when you stop taking it. Doses of 5 mg per day or less are generally considered safe and well tolerated. No case report has documented gynecomastia in an adult male taking melatonin alone.
That said, the research has real gaps. Most clinical trials last weeks to months, and gynecomastia from hormonal shifts can take longer to develop. If you’re taking melatonin alongside other medications known to raise prolactin or lower testosterone, the combined effect could matter more than either drug alone.
Medications More Likely to Cause Gynecomastia
If you’re taking melatonin and noticing breast changes, it’s worth looking at everything else in your medicine cabinet. Drugs with strong, well-documented links to gynecomastia include:
- Anti-anxiety medications like diazepam (Valium)
- Tricyclic antidepressants
- ADHD medications containing amphetamines
- Antiretroviral drugs for HIV, particularly efavirenz
- Opioids used for chronic pain
- Certain antibiotics
Risperidone and similar antipsychotics are especially notable because they are potent prolactin elevators and are sometimes prescribed alongside melatonin for sleep. If you’re on one of these medications, melatonin’s additional prolactin-raising effect could tip the balance.
Supplement Quality Is Another Variable
Melatonin is sold as a supplement in the United States, which means it isn’t regulated the way prescription drugs are. An analysis of 31 over-the-counter melatonin products found wide variation between what the label claimed and what was actually in the pill. More than a quarter of the products tested contained serotonin, some at potentially significant doses. The simplest formulations (plain tablets with melatonin and a basic filler like cellulose) were the most consistent. Products with complex ingredient lists were the least reliable.
This matters because contaminants or unexpected ingredients could produce hormonal effects that get blamed on melatonin itself. If you’re experiencing unusual side effects, switching to a simpler, third-party-tested product is a reasonable step.
Gynecomastia vs. Chest Fat
True gynecomastia involves actual glandular tissue, not just fat. You can often tell the difference with a simple self-check: lying on your back, press your thumb and forefinger together from opposite sides of the breast toward the nipple. If you feel a firm, rubbery disc of tissue centered behind the nipple, that’s glandular tissue. If your fingers slide together smoothly with no resistance until they reach the nipple itself, you’re feeling fat (pseudogynecomastia), which is unrelated to hormones and won’t respond to stopping a supplement.
What Happens if You Stop Taking Melatonin
In the only documented case, breast tissue completely resolved within two days of discontinuing melatonin. The boy’s suppressed testosterone levels also returned to normal. This rapid reversal is consistent with what’s known about prolactin-driven breast changes: when prolactin drops back to normal, early-stage tissue growth typically regresses on its own. Long-standing gynecomastia that has been present for over a year can become fibrotic and may not fully reverse, but this scenario hasn’t been reported with melatonin use.