The short answer
Spain caps over-the-counter melatonin at 1.99 mg per dose. 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) and the European Commission split melatonin products into two regulatory buckets. Up to 1.99 mg per dose: dietary supplement, sold freely in supermarkets and pharmacies. From 2 mg up: prescription-only medicine. The line is drawn by classification, not by safety profile.
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 Journal of Clinical Sleep Medicine paper (Erland & Saxena, 2017) tested 31 American brands: actual melatonin content ranged from 17 to 478 percent of the label, and a quarter contained serotonin as a contaminant. The category is loosely policed.
Spain runs tighter controls. A 1 mg label on an AEMPS-registered brand typically delivers 1 mg within reasonable tolerance.
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 1.99 mg ceiling 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.