How much of orally taken melatonin actually reaches your bloodstream?
On average, only 15% of swallowed melatonin reaches your bloodstream, because the liver breaks down most of it straight away. If you want to achieve higher blood levels, a tablet that dissolves under the tongue may be a better choice.
On average, only about 15% of a swallowed melatonin supplement reaches the bloodstream. Across the studies examined, that figure ranged from 9% to 33%, illustrating how much this varies from person to person and situation to situation.
The reason for this low absorption is what is known as the first-pass effect: the liver breaks down a large proportion of the melatonin before it ever enters the bloodstream. Factors such as age, caffeine, smoking, the contraceptive pill, and whether you eat something at the time of taking it can all influence how much is absorbed.
Because oral absorption is so low and variable, other ways of taking melatonin have been studied. Delivery through the lining of the mouth (under the tongue or against the cheek) appears to bypass the first-pass effect and produces higher blood levels than a standard swallowed tablet. Nasal delivery also shows rapid absorption and higher availability, although very little research in people has been done on this yet. Skin patches give slow and variable absorption, with melatonin partly accumulating in the skin itself. The evidence for all of these alternatives is still limited.
This answer draws on two studies. The data on oral availability (average 15%, range 9–33%) come from one overview study that brought together several investigations into how the body absorbs and processes melatonin. The data on alternative delivery routes come from a separate overview study; the evidence there is thin because very little research in people has been carried out.