★★☆Single study — solid evidence, such as one good trial or a large observational dataset, but not replicated at the top tier.
The decongestant in DayQuil is phenylephrine
Standard DayQuil Cold & Flu gives 10 mg of phenylephrine per adult dose, its only ingredient for a blocked nose. DayQuil Intense Flu contains none.
What the study found
DayQuil's only decongestant is phenylephrine, 10 mg a dose
In standard DayQuil Cold & Flu liquid, phenylephrine is the one ingredient labelled for a blocked nose: 10 mg per 30 mL adult dose, beside acetaminophen for pain and fever and dextromethorphan for cough. Sudafed PE tablets are 10 mg of phenylephrine alone. DayQuil Intense Flu has none.
The caveat · A label is a claim, not a test
A label lists what the maker is permitted to claim under US rules, not proof that the claim holds. Labels change, and this one took effect in June 2026.