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★★☆Single study — solid evidence, such as one good trial or a large observational dataset, but not replicated at the top tier.
Nobody has tested whether phenylephrine clears children's noses
The only study in congested children, 41 aged 2 to 17, measured blood levels after one dose, not relief. The DayQuil label is written for ages 4 and up.
What the study found
No trial has tested whether phenylephrine unblocks children's noses
The one study of oral phenylephrine in congested children, 41 aged 2 to 17, measured blood levels after one dose, not whether their noses cleared. A review of multi-ingredient cold remedies found no evidence of benefit in young children. The DayQuil label is written for ages 4 and up.
The caveat · Mixed remedies, other decongestants
The review's young-children finding covers cold remedies with mixed ingredients, most of them other decongestants, so it is not phenylephrine's own result. The children absorbed the drug much as adults do, with somewhat lower peak levels.