The daily ceiling is 50 mg for sleep and 300 mg for allergy
US sleep-aid labels permit one 50 mg dose of diphenhydramine in 24 hours and say to stop after two weeks. For allergy, the standard clinical reference gives 300 mg a day.
What the study found
One molecule, two daily ceilings, six times apart
Which ceiling applies depends on what the box was sold for. As a sleep aid, US labels permit one 50 mg dose of diphenhydramine in 24 hours and say to stop after two weeks. For allergy, the standard clinical reference gives 300 mg a day, reached with 25 to 50 mg every four to six hours.
The caveat · The 50 mg is one US label's wording
The 300 mg figure comes from a clinical reference chapter for the allergy indication, not from an over-the-counter label, so quoting it about sleep would misdescribe what those products allow. The 50 mg figure is read off one US sleep-aid label, and other markets word their limits differently.