Half a dose is still in the blood nine hours later, longer past 70
A clinical reference gives every four to six hours. In a weight-based study, half a dose was still there at 13.5 hours in people around 70, 9.2 in young adults and 5.4 in children.
What the study found
Half a dose is still there nine hours on, longer if you are old
The four-to-six-hour redosing interval a clinical reference gives is shorter than the drug's own timetable. Half a dose of diphenhydramine is still in an adult's blood nine hours later. In a weight-based study it took 13.5 hours in people around 70, 9.2 in young adults and 5.4 in children.
The caveat · Dosed at about 86 mg, not 25 to 50
Twenty-one people, about seven per group, with standard deviations of 4.2, 2.5 and 1.8 hours around those means. The dose was 1.25 mg per kilogram, roughly 86 mg in the adults, well above the 25 to 50 mg a tablet gives. A separate trial at 25 mg found no age difference at all, most likely because of that gap.