Is melatonin legal in the UK?

Short answer

Melatonin is a prescription-only medicine in the United Kingdom. It is not sold over the counter and it is not classed as a food supplement here, which is the single fact behind almost every search asking whether it is legal.

Why the confusion exists

In the United States melatonin is regulated as a dietary supplement and sits on supermarket shelves. In the UK the MHRA classifies it as a medicinal product, so the same compound that is a grocery item in one country requires a prescription in the other. Nothing about the molecule differs. The difference is entirely regulatory classification, and it is why UK searches for melatonin so often phrase themselves as questions about legality.

What prescription-only actually means

It means supply is controlled, not that possession is criminal. A prescription-only medicine is one a pharmacist may dispense against a valid prescription. That is a supply restriction rather than a possession offence, and the two are routinely conflated in discussion of this compound.

Circadin and the licensed product

The licensed UK melatonin product is a modified-release preparation marketed as Circadin, licensed for a specific indication and age group. Its existence is why melatonin is a medicine here at all: once a compound holds a UK marketing authorisation as a medicine, other presentations of it cannot be sold as supplements.