“It seems like a really simple question, but it’s really difficult,” said Dr. Matthew Rizzo, a professor of neurology at the University of Iowa, who studies driving abilities in people with cognitive impairments caused by conditions like Alzheimer’s disease.

Pharmaceutical companies have long conducted driving tests for certain drugs, but the tests were motivated mainly by marketing goals and not seen as pivotal in getting a drug approved by the F.D.A., said Gary G. Kay, whose company, the Cognitive Research Corporation, leases a driving simulator to pharmaceutical companies. Mr. Kay, for example, helped conduct some of the research showing that the allergy drug Claritin did not cause drowsiness, unlike older antihistamines like Benadryl.

Until recently, the F.D.A. evaluated driving tests by looking at the reaction of the average patient. But that thinking has shifted as research has shown that some people react more strongly to drugs than others, Dr. Farkas said in a telephone interview. “Some people have higher blood levels than others, and some seem to be more sensitive than others,” he said.

In 2011, the agency recognized this variability when it approved the sleep drug Intermezzo, which treats patients who wake up in the middle of the night, in two doses: one for men and a smaller dose for women. Data had shown that the drug’s effects lingered in women for longer than they did in men. The agency’s experience with Intermezzo, along with other research, led it to order in January that the dosage for all drugs containing zolpidem — the active ingredient in Intermezzo, as well as Ambien and related generic sleep drugs — be halved for women.

In the case of suvorexant, the Merck drug, four women had to stop the driving tests because of extreme drowsiness, and the F.D.A. concluded that the drug was linked to an increase of impaired driving among patients who had taken it. Last month, Merck said that the F.D.A. indicated it would eventually approve the drug if the company lowers the proposed doses.

The attention has not been limited to insomnia drugs. In 2011, the F.D.A. approved the drug Horizant to treat restless leg syndrome after the drug makers, XenoPort and GlaxoSmithKline, conducted driving tests because the drug also causes drowsiness. Horizant carries a warning against driving until patients know how the drug affects them, and notes that the effects could last until the morning after it is taken.

Dr. Eric Bastings, the acting director of the F.D.A.’s neurology drug unit, said the agency was writing guidelines to help pharmaceutical companies understand what is expected of them.