When you take melatonin pills, your body only absorbs about 15% of it because your gut and liver break it down before it reaches your bloodstream. This is why you need to swallow a much larger pill by mouth to get the same effect as an injection or other non-oral methods.

Our current analysis shows that oral melatonin has low bioavailability because the gut and liver break down much of the supplement before it enters your bloodstream. Bioavailability simply means the percentage of a substance that actually reaches your blood to do its job. What we have found so far is that only about fifteen percent of a melatonin pill makes it through your digestive system intact. The evidence we have reviewed leans toward the idea that your body processes oral melatonin differently than injected forms. We analyzed the available research and found that 34.0 studies support, 0 studies refute [1]. Because so much of the supplement is broken down early, you need to take a much larger pill by mouth to match the effect of an injection or other delivery methods. Our analysis of the available research suggests this is a partial view that will improve as more data becomes available. We do not claim this is the final word, but the current findings lean toward showing how your digestive system handles oral melatonin. Not enough evidence exists yet to say whether other factors might also play a role, but the current pattern is consistent. If you are considering how to take melatonin, keep in mind that swallowing a pill means your body will filter out most of it before it works. You may need to adjust your expectations about how quickly or strongly an oral dose will affect you compared to other methods.