In one simulator trial, 50 mg drove worse than alcohol at the limit
Forty drivers took 50 mg, a non-sedating antihistamine, enough alcohol to reach about 0.1%, or placebo. Diphenhydramine held the speed of the car ahead least well of the four.
What the study found
Diphenhydramine drove worse than alcohol at the legal limit
Forty drivers drove a simulator on four days: after 50 mg of diphenhydramine, after a non-sedating antihistamine, after enough alcohol to reach about 0.1%, and after placebo. Diphenhydramine gave the worst driving of the four, holding the speed of the car ahead less well than alcohol.
The caveat · Daytime dose, driving an hour later
The dose was daytime, with driving an hour later, the reverse of the bedtime-then-morning pattern a sleep aid creates. On reaction time to a car blocking the lane, alcohol was slowest, so worse than alcohol holds for the measures the paper led with, not every one. The comparator's maker funded the trial.
What we don't know
Fifty milligrams of diphenhydramine impaired driving more than alcohol at roughly the legal limit, and 25 mg looked like placebo. Both are sold as normal doses, so what separates them is a step between two amounts, not a dispute about one.