On a public road, 50 mg made drivers weave more, in eighteen people
Eighteen people drove an instrumented car for 90 minutes, two hours after a dose. Lane position wandered significantly more than on placebo.
What the study found
On a real road, 50 mg of diphenhydramine widened the weave
Eighteen people drove an instrumented car on a public road for 90 minutes, two hours after a dose. Lane position wandered significantly more than on placebo, and mean braking time when the car ahead lit up was 541 milliseconds against 512 on placebo and 474 on a non-sedating antihistamine.
The caveat · Braking gap not shown significant
No p-value is given for diphenhydramine against placebo on braking, so the 29 millisecond gap between them cannot be called significant. Both significant braking comparisons involve the sponsor's own drug. Lane wander is a validated road-test measure and still a proxy for crash risk, not crash risk itself.
What we don't know
The road test adds what a simulator cannot: 50 mg of diphenhydramine made drivers weave on a public road. It tested only 50 mg, so it cannot say whether 25 mg, which matched placebo in another trial, impairs driving.