There is no EU-harmonised rule on melatonin in food supplements. It is governed nationally, and member states differ widely - from open sale at several milligrams to prescription-only status.

Denmark treats melatonin as a medicinal substance. The medicines agency has stated that melatonin is normally found in prescription medicines and may not be added to food supplements. The national guidance on food supplements, issued in June 2024, provides that food supplements may not contain substances or ingredients falling under the medicines act, and adds that a product marketed abroad as a food supplement may be a medicine in Denmark, classified on a concrete assessment of its ingredients or its presentation.

Denmark publishes no numeric melatonin threshold. Unlike Spain, Portugal or Finland, which set figures a supplement may sit beneath, Denmark provides no supplement route at any dose. The bar is categorical rather than quantitative.

It has been applied to ordinary supplements. Two melatonin food supplements sold by companies established outside Denmark, one in the United States and one in Poland, have been classified as unauthorised medicinal products and placed on the agency's published list of medicines illegal to sell in Denmark. Both were classified on the function limb of the medicines definition, meaning the classification followed from the substance's pharmacological effect rather than from anything claimed on the label.