With acetaminophen, about twice the phenylephrine reaches your blood
In single-dose studies of healthy adults, 1,000 mg of acetaminophen with 10 mg of phenylephrine raised blood levels about 2.1-fold. Relief was not tested.
What the study found
Acetaminophen roughly doubled how much phenylephrine reached the blood
In four small studies in healthy adults, 1,000 mg of acetaminophen with 10 mg of phenylephrine raised blood phenylephrine about 2.1-fold, and 500 mg about 1.6-fold, as the two compete for one gut-wall step. A DayQuil adult dose pairs 10 mg of phenylephrine with 650 mg of acetaminophen.
The caveat · Blood levels, not a clearer nose
More phenylephrine in the blood is not shown to mean a clearer nose: no study tested whether the pairing relieves congestion better. The studies gave single doses to healthy volunteers. Their authors work for a company that makes combination painkillers.