Melatonin in the UK: a prescription-only medicine with limited evidence

Melatonin is one of the most widely misunderstood sleep supplements among UK readers, because most online content about it is written for a US audience where it's an over-the-counter dietary supplement. In the UK, melatonin is a prescription-only medicine (POM).

UK regulatory status#

The MHRA classifies all melatonin preparations as POMs. The licensed product is Circadin (2 mg prolonged-release), licensed for short-term treatment of primary insomnia in adults aged 55 and over. NICE has also issued guidance on melatonin use in children with certain neurodevelopmental conditions (under specialist prescription).

Buying melatonin online for personal use, or importing it from the US, sits outside UK medicines law. Quality control of grey-market melatonin is unreliable; independent testing has repeatedly found dose variation of up to 480% from labelled, plus contamination including serotonin and undeclared compounds.

We don't provide dosing guidance for melatonin. Where we describe the research, it's to be honest about what it does and doesn't show.

What the evidence supports#

Jet lag. Melatonin is widely used around long-haul travel, but neither study cited on this page assessed jet lag, so we don't give an effect size here.

Delayed sleep-wake phase disorder (DSWPD). Reasonable evidence for advancing the circadian phase in adolescents and adults with diagnosed DSWPD. Used clinically under specialist supervision.

Specific paediatric indications. NICE supports specialist-prescribed melatonin in children with neurodevelopmental disorders and persistent insomnia, where behavioural approaches have failed.

Modest effect on sleep onset latency in primary insomnia. Around 7-minute average reduction in meta-analyses, statistically significant, clinically modest.

What the evidence does not support#

• Major improvements in sleep duration or quality in healthy adults with no sleep disorder. The effect is small at best and the trials are heterogeneous.
• Antioxidant, anti-aging, cancer-prevention claims are mechanistic hypotheses with weak human evidence.
• High-dose chronic use: most US over-the-counter doses (3–10 mg) are far above the doses studied (0.3–2 mg) and may produce next-day grogginess without adding benefit.

Why it gets so much attention#

Melatonin is the body's main signal for "darkness" and "biological night". The marketing leans heavily on the biology — accurately, in part. The leap from "melatonin signals night" to "swallowing 5 mg of melatonin meaningfully improves sleep in healthy adults" is the leap the evidence doesn't strongly support.

Most of what makes night sleep easier is environmental: light, temperature, timing. Pharmacological melatonin is not a substitute for those, and in healthy adults its effect is small.

What we'd actually say#

If you have persistent insomnia, the NHS first-line is CBT-I (often via the Sleepio app under NHS prescription in some areas), not melatonin. If your sleep timing is severely off (DSWPD), a sleep clinic referral is the right route, where prescribed melatonin may be appropriate.

For jet lag, talk to a GP if you travel across many time zones frequently. Timing your light exposure at the destination and allowing a few days to adjust also helps.

When to talk to a clinician#

• Persistent insomnia (>3 nights/week for >3 months)
• Severely shifted sleep schedule that doesn't respond to behavioural change
• Children with neurodevelopmental disorders and significant sleep disruption: specialist territory
• Considering melatonin while on antidepressants, anticoagulants, or contraceptives (interactions exist)
• If you've been importing melatonin and want to discuss safe use with a clinician

The honest summary: melatonin is real biology, real medicine, modest effects, and POM in the UK for good reasons. Most of the online discussion is from a different regulatory and clinical context.