Melatonin is generally well tolerated at low doses, but it does come with a real list of side effects that range from next-day grogginess to vivid nightmares. Because melatonin is sold as a dietary supplement in the U.S. rather than regulated as a medication, many people assume it’s completely harmless. The reality is more nuanced, especially if you take it regularly, use higher doses, or are on other medications.
Common Side Effects
The most frequently reported side effect of melatonin is daytime drowsiness. This might sound obvious for a sleep aid, but the drowsiness can linger well into the morning and affect your alertness at work or behind the wheel. Other common side effects include headache, dizziness, and nausea.
Less common but still reported effects include irritability, reduced alertness during the day, short-term feelings of mild depression, stomach cramps, and dry mouth. Most of these resolve once you stop taking melatonin or lower the dose. The NHS recommends talking to a doctor if you’re taking more than 2 mg, which suggests that many side effects become more pronounced at higher doses.
Vivid Dreams and Nightmares
One of the more unsettling side effects is a noticeable increase in vivid or disturbing dreams. This isn’t random. Melatonin increases the amount of time you spend in REM sleep, the sleep stage where the most intense dreaming occurs. More time in REM means more opportunity for vivid, emotionally charged, or outright bizarre dreams.
The mechanism likely involves vasotocin, a brain chemical that increases alongside melatonin levels. Higher vasotocin promotes more REM sleep, which can tip the balance toward nightmares in some people. If you’re already prone to bad dreams or have anxiety-related sleep disturbances, supplemental melatonin may make them worse.
Interactions With Other Medications
Melatonin interacts with a surprisingly wide range of medications. Some of these interactions are potentially serious:
- Blood thinners. Melatonin can reduce blood clotting, so combining it with anticoagulant medications or supplements that thin the blood may increase your risk of bleeding.
- Blood pressure medications. Melatonin can worsen blood pressure control in people already on blood pressure drugs.
- Seizure medications. Melatonin may reduce the effectiveness of anticonvulsants and increase seizure frequency, particularly in children with neurological conditions.
- Sedatives and sleep aids. Taking melatonin alongside other sedating drugs creates a compounding effect, leading to excessive drowsiness.
- Birth control pills. Hormonal contraceptives combined with melatonin can amplify sedation and increase the likelihood of other melatonin side effects.
- Immune-suppressing drugs. Melatonin can stimulate immune function, which directly works against the purpose of immunosuppressant therapy (used after organ transplants or for autoimmune diseases).
- Certain antidepressants. Fluvoxamine, a medication used for obsessive-compulsive disorder, can dramatically raise melatonin levels in the body, causing excessive drowsiness.
If you take any prescription medication, it’s worth checking whether melatonin could interfere before adding it to your routine.
Risks for Older Adults
Melatonin is often marketed as a gentler alternative to prescription sleep aids for older adults, but it carries its own concerns. A matched cohort study published in the journal Age and Ageing found that people taking melatonin had a 44% higher risk of fractures after adjusting for other factors. This elevated risk appeared mainly in people who had received three or more prescriptions, suggesting that longer or repeated use drives the danger.
The connection is straightforward: melatonin causes drowsiness, and drowsiness increases the chance of falling. For someone over 65, a fall is far more likely to result in a serious fracture. While melatonin doesn’t appear to impair motor function the way traditional sleep medications do, the sedation alone is enough to raise concern.
Concerns for Children
Melatonin use in children has surged in recent years, which prompted the American Academy of Sleep Medicine to issue a health advisory. Their core concern isn’t necessarily the hormone itself but the product. In the U.S., melatonin supplements are not subject to FDA oversight, and the actual melatonin content in supplements can vary wildly. One study found that the melatonin in commercially available products ranged from less than half to more than four times the amount listed on the label. The worst variability was found in chewable tablets, the form children are most likely to take. Some products even contained other chemicals that typically require a prescription.
The AASM recommends that parents look for supplements carrying the USP Verified Mark, which confirms the product contains what the label claims. Only a small number of melatonin products have earned this mark. As for the fear that melatonin could delay puberty, a two-year study of children and adolescents (ages 2 to 17) taking melatonin nightly found no evidence of delayed growth, abnormal weight changes, or disrupted pubertal development.
What About Long-Term Use?
Most melatonin research has studied short-term use, typically a few weeks to a few months. The safety picture for long-term, daily use is less clear and more concerning. A study of adults with insomnia who took melatonin for at least a year found an 89% higher risk of developing heart failure, a threefold increase in heart failure-related hospitalizations, and double the rate of death from any cause over a five-year follow-up period. This was an observational study, meaning it can’t prove melatonin caused those outcomes (people with worse insomnia may have other health risks), but the size of the association is hard to ignore.
There’s also the open question of whether taking melatonin nightly causes your body to produce less of it naturally. Research hasn’t provided a definitive answer on this yet, but many sleep specialists recommend using melatonin for specific, time-limited purposes (jet lag, shift work adjustment, a temporary sleep disruption) rather than as an indefinite nightly habit.
Supplement Quality Is Part of the Problem
A significant portion of melatonin’s side effect profile may have nothing to do with melatonin itself. When researchers tested ten commercially available melatonin supplements in the U.S., 40% failed to meet acceptable content standards, meaning the actual melatonin inside deviated significantly from what the label promised. Several products also contained elevated levels of unidentified impurities, with some approaching 1% of the total content.
This means you might be taking far more melatonin than you intend, or you might be ingesting unknown contaminants alongside it. If you experience unexpected side effects, the product itself could be a factor. Choosing a supplement that has been independently verified for purity (the USP Verified Mark is the gold standard in the U.S.) reduces this risk considerably, though very few melatonin products currently carry it.
Dose Matters More Than Most People Think
Melatonin supplements commonly come in doses of 3 mg, 5 mg, and 10 mg, but the amount your body naturally produces each night is a fraction of a milligram. Many sleep specialists suggest starting at 0.5 to 1 mg and only increasing if that doesn’t help. Higher doses don’t necessarily produce better sleep, and they do increase the likelihood and severity of side effects like morning grogginess, headaches, and vivid dreams.
Taking a higher dose can also shift the timing of your internal clock in ways you didn’t intend, making it harder to fall asleep at your target time on subsequent nights. If you’ve been taking 5 or 10 mg and experiencing side effects, try cutting back before assuming melatonin doesn’t agree with you. The problem may be the dose, not the supplement.