Melatonin is primarily metabolized in the liver, where enzymes convert roughly 90% of it into a single metabolite called 6-hydroxymelatonin. This intermediate is then quickly tagged with a sulfate group to become 6-sulfatoxymelatonin, which is excreted in urine. The entire process is fast: melatonin’s elimination half-life is about 40 to 55 minutes, meaning most of it clears your bloodstream within a few hours.
The Liver Does Most of the Work
The liver is melatonin’s main processing center. A family of enzymes called cytochrome P450 breaks melatonin down through two chemical reactions. The dominant one, aromatic hydroxylation, clips a hydroxyl group onto melatonin’s ring structure at a specific position, producing 6-hydroxymelatonin. The second, less common reaction, called O-demethylation, strips a methyl group from melatonin and converts it into N-acetylserotonin (essentially returning it to a precursor form).
The enzyme CYP1A2 handles the bulk of this work. CYP1A1 and CYP1B1 contribute as well, and all three favor the hydroxylation pathway. A separate enzyme, CYP2C19, selectively performs the O-demethylation reaction. Once 6-hydroxymelatonin is produced, the liver conjugates it with sulfate to form 6-sulfatoxymelatonin. This conjugated form is water-soluble and passes easily into urine, which is how the vast majority of melatonin leaves the body.
Metabolism Outside the Liver
Smaller amounts of melatonin are broken down in the skin, lungs, intestines, and brain. These tissues express some of the same CYP enzymes found in the liver, particularly CYP1B1, which performs 6-hydroxylation in places the liver doesn’t reach. The skin is especially interesting because it uses at least three different metabolic pathways: the same hydroxylation pathway the liver uses, a pathway that produces compounds called AFMK and AMK (part of the kynuric pathway), and a non-enzymatic route triggered by UV light.
That UV-driven breakdown means sunlight exposure can directly degrade melatonin in skin cells, producing 6-hydroxymelatonin, 2-hydroxymelatonin, and possibly 4-hydroxymelatonin without any enzymes involved. This ties into melatonin’s role as a local antioxidant in skin tissue.
Melatonin’s Antioxidant Cascade
When melatonin neutralizes free radicals, it doesn’t just disappear. It breaks down into metabolites that are themselves antioxidants, creating a chain reaction. The two most studied products of this non-enzymatic breakdown are AFMK and AMK, both of which can scavenge additional reactive molecules and reduce inflammation. This means a single melatonin molecule can neutralize multiple free radicals as it degrades through successive metabolites, a property that makes it unusually efficient compared to most antioxidants.
Why Oral Melatonin Levels Vary So Much
If you take melatonin as a supplement, a large and unpredictable portion of it gets destroyed before it ever reaches your bloodstream. After you swallow a tablet, melatonin passes through the gut wall and into the liver before entering general circulation. This “first-pass” metabolism chews through most of the dose. In one study of healthy men given 500 micrograms orally, peak blood levels varied by nearly 20-fold between individuals, ranging from 480 to 9,200 nanograms per liter. That’s an enormous spread from the same dose, and it’s driven largely by individual differences in liver enzyme activity.
The oral elimination half-life averages about 54 minutes, while intravenous melatonin (which skips the liver on first pass) clears slightly faster at about 39 minutes. Previous research across various doses has found oral half-lives between 46 and 65 minutes and intravenous half-lives between 28 and 60 minutes.
Drugs That Slow Melatonin Breakdown
Because CYP1A2 is the primary enzyme responsible for clearing melatonin, anything that inhibits this enzyme will cause melatonin to accumulate in the blood. The most dramatic example is fluvoxamine, an antidepressant that blocks CYP1A2 so effectively that it increases melatonin exposure by roughly 17-fold and peak blood levels by 12-fold. That combination is considered one to avoid. Fluoroquinolone antibiotics also inhibit CYP1A2 and can raise melatonin levels, though less dramatically. Estrogen-containing medications, including hormonal contraceptives and hormone replacement therapy, slow melatonin clearance by inhibiting both CYP1A1 and CYP1A2, which partly explains why women on these medications sometimes report changes in sleep patterns.
Conversely, substances that speed up CYP1A2 activity, like cigarette smoke, can accelerate melatonin breakdown and reduce its effectiveness.
Liver Disease Changes the Equation
Since the liver handles 90% of melatonin metabolism, liver damage significantly alters melatonin levels. People with cirrhosis show elevated melatonin throughout the day and a delayed nighttime peak. Their livers simply can’t clear melatonin at normal rates, so it lingers in the bloodstream longer. The melatonin response to light exposure also appears blunted in cirrhosis, likely because the impaired liver can’t ramp up clearance the way a healthy liver does when light signals the pineal gland to stop producing melatonin. This sluggish clearance contributes to the disrupted sleep-wake cycles that are common in advanced liver disease.
How Melatonin Leaves the Body
The final step is straightforward. About 90% of melatonin’s breakdown products exit through urine, primarily as 6-sulfatoxymelatonin. This metabolite is so reliably linked to melatonin production that researchers and clinicians use urinary 6-sulfatoxymelatonin levels as a proxy for how much melatonin your body made (or absorbed from a supplement) over a given period. A smaller fraction of metabolites leaves through bile and feces, but the kidneys handle the overwhelming majority.