Circadin tablets have a special prolonged release formulation that delivers the melatonin dose over 8-10 hours. The melatonin you can buy in health food shops is released immediately in to the system in one go. This immediate release of large doses of melatonin does not mimic your body's normal hormone levels and is less likely to be effective and may be more likely to produce side effects.
Circadin is a medicine and has a product license. This means that it has undergone testing and is subject to standards set by the MHRA (Medicines Health Regulatory Authority) in the UK. As such it is mandatory to have a batch number and expiry date and be subject to strict manufacturing regulation. An audit trail from manufacture to consumption is traceable and thus ensures it contains what it states. Health foods are not subject this strict regulation.