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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.
The drowsiness comes with a dry mouth, a slow gut and blurred vision
Diphenhydramine blocks acetylcholine as well as histamine, so at ordinary doses it dries the mouth, slows the bowel, makes urine harder to pass and blurs vision.
What the study found
The drowsiness arrives with a dry mouth and a slowed gut
These come at ordinary doses, not overdose. As well as histamine, which is what makes it sedating, diphenhydramine blocks acetylcholine, drying the mouth, slowing the bowel, making urine harder to pass and blurring vision. The drowsiness is one item on that list, not a separate thing.
The caveat · No rate given for any effect
The source is a clinical reference chapter summarising other work rather than a study. It names these effects without giving a rate for any of them, so this says what the drug does at ordinary doses, not how likely each is in a given person.