The short answer
In Spain, a melatonin food supplement must provide less than 2 mg a day: above that, the AEMPS treats melatonin as a medicine. In the US, any grocery store carries 5, 10, even 20 mg bottles. The intuitive read – more dose, more effect – falls apart once you open the trials. For mild insomnia and for circadian-rhythm tuning, low doses work, and sometimes work better than high ones.
Why 1.99 mg and not 2
The Spanish Agency of Medicines and Medical Devices (AEMPS) treats melatonin as a medicine when a product provides more than 2 mg a day, and a food supplement must stay below that amount. What counts is the daily dose recommended on the label, not a single capsule: two 1.99 mg doses add up to 3.98 mg, which already puts the product in medicine territory (Basque Government Health Department, 2013). The 1.99 mg figure appears in no regulation: it is how manufacturers stay just under the threshold. The EU has set no common maximum for melatonin in supplements. The line is drawn by classification, not by safety profile.
Being a medicine does not always mean a prescription. Spanish pharmacies sell 3 mg and 5 mg melatonin medicines without a prescription, authorised for jet lag in adults, while prolonged-release 2 mg melatonin is prescription-only.
In the EU, prolonged-release 2 mg formulations such as Circadin are licensed for insomnia in adults over 55. Below that threshold, regulation eases off because the product is framed as nutritional rather than pharmacological.
What the trials say about dose
Two findings from the clinical literature worth remembering:
- Brzezinski et al., 2005 (Sleep Medicine Reviews) – meta-analysis of 17 RCTs. Melatonin shortens sleep latency by an average of 4 minutes. Effect size does not scale with dose: 0.5 mg and 5 mg deliver similar results.
- Zhdanova et al., 2001 – 0.3 mg restored sleep in older adults with insomnia. Higher doses saturate receptors and keep plasma levels elevated into the morning, which links to next-day grogginess.
A clinically important number: overnight endogenous melatonin in plasma sits around 60–100 pg/mL. A 0.3 mg capsule raises that peak roughly tenfold. A 5 mg capsule pushes it 50 to 150 times above the physiological range. Higher doses do not produce more sleep. They produce supraphysiological levels that linger past sunrise.
What melatonin actually treats
The strongest evidence is not for insomnia itself but for circadian-rhythm correction. Three clean indications:
- Jet lag – 0.5 mg taken a few hours before local bedtime for the first 2–4 nights. Cochrane reviewed nine trials: consistent benefit.
- Shift work – evidence is thinner, but the effect holds.
- Delayed sleep-phase disorder – teens and young adults who fall asleep at 3 a.m. Low doses 4–6 hours before the target bedtime move the cycle forward.
For ordinary chronic insomnia, a 4-minute average effect means melatonin is a limited tool, not a fix. It works alongside sleep hygiene, and for persistent disorders alongside cognitive behavioural therapy for insomnia (CBT-I).
Where the US market gets it wrong
In the US, 5–10 mg bottles are the norm. A team at the University of Guelph in Canada tested 31 supplements (Erland & Saxena, Journal of Clinical Sleep Medicine, 2017): actual melatonin content ranged from 83% below to 478% above the label, and eight of them, about a quarter, contained serotonin. The category is loosely policed.
In Spain, melatonin above 2 mg a day can only be sold as a medicine authorised by the AEMPS. Food supplements need no prior authorisation: the company notifies the health authorities when the product goes on sale (Royal Decree 1487/2009, article 9).
Side effects and cautions
High doses produce morning headache, residual drowsiness and vivid dreams. Melatonin is not recommended in pregnancy or lactation due to thin data. Notable interactions involve anticoagulants, anticonvulsants and immunosuppressants. Melatonin is metabolised by CYP1A2 and can shift the kinetics of several drugs taken alongside it.
In people with depression, preliminary data suggest that high doses may worsen symptoms – another reason to start low.
How to use it sensibly
- Start at 0.3 to 0.5 mg, 30–60 minutes before bed, and stay there if it works.
- For jet lag: the same low dose a few hours before destination bedtime.
- Do not run it past 4 weeks without revisiting the indication.
- Melatonin does not replace sleep hygiene. Screens, late coffee and a lit bedroom outweigh any supplement.
Bottom line
Spain's supplement ceiling, under 2 mg a day, looks restrictive but tracks what the literature recommends. The high-dose bottles dominating US shelves move product, not sleep. When melatonin helps, it helps at low doses in narrow scenarios. The rest is regulatory noise and well-packaged placebo.