One trial: pseudoephedrine eased congestion, phenylephrine did not
One dose each, in 39 people with hay fever in a pollen chamber: pseudoephedrine 60 mg eased congestion over 6 hours, and phenylephrine 12 mg did no better than placebo.
What the study found
Pseudoephedrine beat phenylephrine and placebo in one pollen chamber
In a pollen chamber, 39 people with grass allergy took one dose each of phenylephrine 12 mg, pseudoephedrine 60 mg and a placebo, on separate days. Over 6 hours pseudoephedrine eased congestion more than both, and airflow measures agreed. Phenylephrine could not be told apart from placebo.
The caveat · Earlier doses may have carried over
Each person took all three in turn, and the authors flag that earlier doses may have carried over: in the first round alone, phenylephrine's gap over placebo was 64% of pseudoephedrine's, against 17% across all rounds. It tested a single dose. The congestion came from pollen, not a cold.