Melatonin Side Effects: What to Expect and What Warrants a Closer Look

What side effects does melatonin commonly cause?

Melatonin works by signaling to the brain that night has arrived. At the right dose and timing, that signal is useful. At higher doses — and many over-the-counter products contain doses well above what clinical research typically tests — the signal overshoots, leaving residual drowsiness the next morning, a mild headache, nausea, or dizziness 2Ref 2National Center for Complementary and Integrative Health (2024).Melatonin: What You Need To Know.NIH overview of melatonin safety: common side effects (headache, dizziness, nausea, drowsiness), drug interactions (blood thinners, epilepsy medications), vulnerable groups (pregnant/breastfeeding, older adults, people with dementia), and the unestablished long-term safety profile. These are the most frequently reported side effects.

They are generally dose-related: taking less, or taking it earlier in the evening, often reduces them. Extended-release formulations have different absorption timing than immediate-release, which changes both the effectiveness and side-effect profile 1Ref 1Herxheimer A, Petrie KJ (2002).Melatonin for the Prevention and Treatment of Jet Lag.Melatonin pharmacology including dose, timing, and the side effects reported in clinical trials of exogenous melatonin.

Are there less common effects worth knowing about?

Some people report vivid or unusual dreams after starting melatonin, which relates to how it influences REM sleep. Mild irritability or mood changes have been noted occasionally. At higher doses, a transient drop in alertness can feel disorienting.

Because melatonin is a hormone — not simply a sedating herb — it could theoretically interact with other hormonal systems, though at typical supplement doses this is not well-documented in healthy adults. The long-term effects of daily melatonin use remain understudied, which is a meaningful gap in the evidence 2Ref 2National Center for Complementary and Integrative Health (2024).Melatonin: What You Need To Know.NIH overview of melatonin safety: common side effects (headache, dizziness, nausea, drowsiness), drug interactions (blood thinners, epilepsy medications), vulnerable groups (pregnant/breastfeeding, older adults, people with dementia), and the unestablished long-term safety profile.

An additional practical concern: a study of 31 commercial melatonin supplements found that more than 71% did not meet label claims within a 10% margin, and 26% contained unlabeled serotonin — a related compound that can cause harmful effects at significant levels 5Ref 5Erland LAE, Saxena PK (2017).Melatonin Natural Health Products and Supplements: Presence of Serotonin and Significant Variability of Melatonin Content.Of 31 commercial melatonin supplements tested, 71% failed to meet label claims within 10%; 26% contained unlabeled serotonin — a quality and safety concern for consumers.

Which medications interact with melatonin?

Melatonin can amplify the effect of blood-thinning medications such as warfarin, potentially raising bleeding risk 2Ref 2National Center for Complementary and Integrative Health (2024).Melatonin: What You Need To Know.NIH overview of melatonin safety: common side effects (headache, dizziness, nausea, drowsiness), drug interactions (blood thinners, epilepsy medications), vulnerable groups (pregnant/breastfeeding, older adults, people with dementia), and the unestablished long-term safety profile. It may affect blood sugar regulation in people taking diabetes medications. Immunosuppressants and melatonin may interact in ways that alter immune function. Some anticonvulsants and certain antidepressants — particularly fluvoxamine — can significantly raise blood melatonin levels.

If you take any prescription medications, checking with your pharmacist before starting melatonin is genuinely important, not a formality.

Who needs extra care with melatonin?

Children: Melatonin is widely used in children with sleep difficulties and neurodevelopmental conditions such as ADHD and autism spectrum disorder, often with clinician guidance. Long-term safety data in children is very limited, and appropriate doses differ significantly from adult use. A pediatrician should be involved before giving melatonin to a child.

Pregnant and breastfeeding individuals: Safety data are limited; endogenous melatonin plays roles in fetal development that are not fully understood. Consult your OB before using it 2Ref 2National Center for Complementary and Integrative Health (2024).Melatonin: What You Need To Know.NIH overview of melatonin safety: common side effects (headache, dizziness, nausea, drowsiness), drug interactions (blood thinners, epilepsy medications), vulnerable groups (pregnant/breastfeeding, older adults, people with dementia), and the unestablished long-term safety profile.

Older adults: Melatonin may be metabolized more slowly with age, increasing next-day grogginess and fall risk. Lower starting doses and careful timing matter more in this group 3Ref 3National Institute on Aging (2023).Sleep and Older Adults.Older adults metabolize medications including melatonin more slowly, raising the risk of next-day grogginess and fall risk at doses tolerated by younger adults.

Is melatonin safe for long-term daily use?

Short-term use appears safe for most adults, but long-term safety data is limited 2Ref 2National Center for Complementary and Integrative Health (2024).Melatonin: What You Need To Know.NIH overview of melatonin safety: common side effects (headache, dizziness, nausea, drowsiness), drug interactions (blood thinners, epilepsy medications), vulnerable groups (pregnant/breastfeeding, older adults, people with dementia), and the unestablished long-term safety profile. For chronic insomnia, Cognitive Behavioral Therapy for Insomnia (CBT-I) is the evidence-based first-line treatment recommended over ongoing supplementation 4Ref 4Edinger JD, Arnedt JT, Bertisch SM, et al. (2021).Behavioral and Psychological Treatments for Chronic Insomnia Disorder in Adults: An American Academy of Sleep Medicine Clinical Practice Guideline.CBT-I as the evidence-based first-line treatment for chronic insomnia, preferred over ongoing melatonin supplementation for long-term use. If you have been using melatonin regularly for weeks or months without clear benefit, a clinician visit is worthwhile — both to evaluate the underlying sleep problem and to discuss whether continued use makes sense.