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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.
Phenylephrine alone has no published modern test in real colds
All four modern placebo trials were in hay fever. Cold trials test multi-ingredient remedies, which ease some symptoms without showing what phenylephrine adds.
What the study found
The published modern trials were all in hay fever, not colds
All four modern placebo trials of oral phenylephrine were in hay fever. In colds, the products tested are mixes: pooled antihistamine-decongestant trials cut treatment failure, and a capsule with 4 mg of phenylephrine eased overall symptoms. Neither shows what phenylephrine added.
The caveat · The one cold trial stopped early
The one modern placebo trial begun in real colds was stopped early for low enrolment and never published in a journal. Older single-dose laboratory studies, from the 1950s to 1970s, were never published either.