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.
Sweden sits at the strictest end of that range, and it is one of the clearest national positions in Europe. The Swedish medicines agency states that melatonin is classified as a medicine in Sweden, may not be sold as a food supplement, and may only be sold in pharmacies.
Non-prescription melatonin exists, but within narrow limits. The agency's substance report for over-the-counter melatonin restricts the non-prescription indication to short-term treatment of jet lag in adults over 18, with a maximum daily dose of 5 mg and pack sizes of up to ten tablets. The same report states that sale in non-pharmacy retail is not accepted in Sweden, because of the risk of off-label use. Higher strengths and any use other than jet lag remain prescription-only.
The consequence for a cross-border supplement seller is unambiguous. There is no dose at which melatonin may be sold into Sweden as a food supplement, and there is no retail channel outside pharmacies through which it may be sold at all.