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★★★Well replicated — this finding’s sources average out to meta-analyses, systematic reviews or high-powered trials.
Even four times the usual dose did no better than placebo
In one week-long hay-fever trial, doses up to 40 mg every 4 hours eased congestion no more than a placebo did.
What the study found
Up to four times the labelled dose did no better than placebo
In one week-long trial, 539 adults with hay fever took 10, 20, 30 or 40 mg of phenylephrine every 4 hours, up to four times the labelled 10 mg dose, or a placebo. Congestion eased about equally in every group. FDA advisers agreed unanimously that higher doses are not supported either.
The caveat · Unblinded, by the trial's own title
The trial's own title calls it open-label, so people may have known what they were taking. That usually inflates a self-rated benefit rather than hiding one. Everyone in it had hay fever, not a cold.