Taking 15 mg of melatonin is not dangerous for most adults, but it’s roughly 30 times the dose your body produces naturally and well above the 0.5 to 5 mg range typically used as a sleep aid. At this level, you’re likely to experience side effects that go beyond simple drowsiness, and the dose interacts meaningfully with several common medications.
What You’ll Likely Feel
A systematic review of melatonin safety found that doses above 10 mg per day increase the rate of minor side effects compared to lower doses. The most common ones at this level are excessive drowsiness, headache, and dizziness. Some people also report vivid or disturbing dreams, mild confusion, nausea, and a drop in body temperature that can leave you feeling chilly.
These effects tend to hit harder than what you’d notice at a standard 3 or 5 mg dose, mostly because melatonin’s sedating properties scale with the amount you take. You may feel groggy well into the next morning, especially if you took it late at night. The drowsiness can be significant enough to impair driving or other tasks that require alertness.
In clinical trials comparing high-dose melatonin (20 to 25 mg) to placebo, participants were about 40% more likely to report adverse effects. However, the rate of serious adverse events didn’t increase. The side effects were uncomfortable but not medically dangerous for otherwise healthy adults.
Is 15 mg an Overdose?
Not in any clinical sense. Melatonin is one of the least toxic supplements available. In one documented case, a woman was brought to the emergency department after intentionally ingesting 120 mg of melatonin (60 tablets of 2 mg each). She was monitored for 12 hours and discharged without complications. At 15 mg, you’re nowhere near a threshold that would cause organ damage or require emergency treatment.
That said, “not toxic” doesn’t mean “no consequences.” Your body normally produces a tiny fraction of a milligram of melatonin each evening. Flooding your system with 15 mg can paradoxically make sleep worse for some people by disrupting the timing signals your brain relies on. Instead of a clean on-off signal, your brain gets an overwhelming dose that can shift your sleep-wake cycle in unpredictable ways, leaving you alert at odd hours the following day.
Effects on Your Body’s Own Melatonin
A common concern is whether taking a large dose will shut down your body’s natural melatonin production over time. The evidence suggests it doesn’t. In one study, even a subject who took 50 mg of melatonin nightly for 37 consecutive days showed no change in their natural melatonin production once they stopped. A separate trial with night shift workers taking melatonin for seven days found the same result: the body’s own melatonin output was unchanged on the eighth day.
What high doses can do is shift the timing of your internal clock. Your brain uses melatonin as a phase marker, so a large external dose can push your natural rhythm earlier or later. This is why you might feel “off” for a day or two after a single high dose, even though your production capacity remains intact.
Medication Interactions at Higher Doses
Where 15 mg becomes more concerning is if you take other medications. The interactions that exist at any melatonin dose become more pronounced at higher amounts.
- Blood thinners. Melatonin reduces blood clotting. Combining it with anticoagulant medications increases bleeding risk.
- Blood pressure medications. Melatonin can worsen blood pressure in people already taking blood pressure drugs, potentially counteracting the medication.
- Sedatives and sleep medications. Melatonin adds to the sedative effect of other central nervous system depressants, including benzodiazepines and certain antidepressants. At 15 mg, this stacking effect can cause profound drowsiness.
- Seizure medications. Melatonin may reduce the effectiveness of anticonvulsants, particularly in children with neurological conditions.
- Diabetes medications. Melatonin affects blood sugar levels and can interfere with glucose management.
- Fluvoxamine. This OCD medication blocks the enzyme that breaks down melatonin, which means your body clears it much more slowly. Taking 15 mg while on fluvoxamine could result in melatonin levels far higher than intended.
- Birth control pills. Hormonal contraceptives can amplify melatonin’s sedative effects and increase the likelihood of side effects.
Why High Doses Are Sometimes Used
While 15 mg is excessive for basic sleep support, doses in this range do appear in clinical research. Studies have used 10 to 20 mg daily for cancer patients receiving immunotherapy, with one trial reporting disease stabilization and improved quality of life in about 40% of participants with metastatic tumors. Other research has explored doses of 20 mg daily in patients with AIDS, finding generally beneficial effects on immune function. In these contexts, melatonin is being used for its antioxidant and immune-modulating properties rather than as a sleep aid, and patients are under medical supervision.
For sleep specifically, research consistently shows that lower doses work just as well. Studies using doses from 2 to 20 mg found improvements in sleep quality and how quickly people fell asleep, but the benefits don’t scale proportionally with dose. Most sleep researchers consider 0.5 to 3 mg the sweet spot for triggering sleepiness without next-day grogginess.
What Your Supplement May Actually Contain
One underappreciated factor with a 15 mg melatonin product is label accuracy. Melatonin is sold as a dietary supplement in the United States, which means it doesn’t go through the same manufacturing controls as prescription drugs. Independent testing has repeatedly found that melatonin supplements contain anywhere from a fraction of the labeled dose to several times more than what’s listed. If you took a product labeled 15 mg, the actual amount in your system could be meaningfully different in either direction.
This variability matters more at higher doses because the margin for unwanted effects narrows. A product that contains 150% of its labeled dose would put you at 22.5 mg, pushing into territory where side effects become more common and more noticeable.
If You Already Took 15 mg
You’ll most likely feel very drowsy and possibly a bit dizzy or foggy. Stay in a safe environment where you can sleep it off. Avoid driving. The effects typically wear off within 6 to 8 hours, though you may feel sluggish the next morning. Drink water, as some people experience mild nausea. There’s no need to try to counteract the dose or induce vomiting.
If you’re taking any of the medications listed above, pay closer attention to how you feel over the next several hours. Unusual bleeding, significant drops in alertness, or changes in blood sugar are worth monitoring. For otherwise healthy adults, a single 15 mg dose is an uncomfortable experience, not a medical emergency.