The top police officer in Chicago told the mayor he had “a couple of drinks with dinner” before he fell asleep at a stop sign while driving home on Thursday, an incident the chief contends was related to a change in his blood pressure medication.

Superintendent Eddie Johnson did not mention having anything to drink when he spoke to reporters on Thursday night and a department spokesman said the officers who responded to a 911 call reporting a man asleep in a car at a stop sign did not observe any signs of impairment.

Mayor Lori Lightfoot told the Chicago Sun-Times on Friday she agreed with Eddie Johnson’s decision to request an internal affairs investigation.

“It was the right thing,” Lightfoot said. “We’ll see how that plays itself out.”

The police spokesman Anthony Guglielmi said: “While we have no indication of impropriety at this time, this question can only be answered by the internal affairs investigation.”

Guglielmi said on Thursday: “There were no charges of intoxication, no information of intoxication as far as I know.”

Johnson told reporters on Thursday evening he was driving home at about 12.30am after having let his driver go home, when he felt lightheaded. He said he pulled over and fell asleep.

Officers allowed him to drive home and did not administer a breathalyzer or a field sobriety test. Johnson said officers do such tests only when a motorist appears impaired or officers smell alcohol or cannabis.

He said the medical episode was the result of not following doctor’s orders.

“When he adjusted my medication, I took the old medication for high blood pressure but I failed to put the new medication in,” he said.

During a news conference with the then mayor, Rahm Emanuel, in early 2017, Johnson had to be helped to a seat after he appeared to be about to fall.

He was diagnosed decades ago with a kidney condition, which two years ago led to a transplant with his son as the donor. This year he was hospitalized for a blood clot.

Mayor Lightfoot said Johnson told her he had just changed medication, and said she has “no reason to doubt” his account.

“We know he’s had some medical issues,” she said. “He’s on the other side of a kidney operation, which is obviously very, very serious. There have been some issues with high blood pressure, and so forth.”

She also said that she knew from dealing with her parents that certain medications have side effects. “So I take him at his word.”

“He’s a grown man,” she added. “He had a couple of drinks with dinner.”