★★☆Single study — solid evidence, such as one good trial or a large observational dataset, but not replicated at the top tier.
Your gut breaks down much of a phenylephrine pill on the way in
Taken by mouth, phenylephrine is extensively converted in the gut wall, and FDA advisers saw that low uptake as consistent with trials finding no relief.
What the study found
Swallowed phenylephrine is heavily broken down in the gut wall
Taken by mouth, phenylephrine is extensively converted in the gut, through a step in the gut wall, before it reaches the blood. FDA advisers read that low bioavailability, the small share of a dose reaching the blood unchanged, as consistent with the trials finding no relief.
The caveat · Never measured inside the nose
No study has measured phenylephrine in the lining of the nose, where it would have to act. So the link between little drug in the blood and no relief is inferred, not shown.