(CNN) -- Kerry Kennedy, daughter of the late Sen. Robert F. Kennedy and ex-wife of New York Gov. Andrew Cuomo, took the stand Wednesday in a DWI trial stemming from a 2012 traffic accident and repeated her claim that she mistakenly took a sleeping pill the morning of the crash.
She said that she never felt the effects of the powerful sleep aid in her system.
Kennedy is facing a misdemeanor charge of driving while intoxicated for allegedly swerving between lanes and careening into a tractor-trailer on a Westchester County, New York, interstate in July 2012. She drove away from the accident scene despite a shredded tire, and was found collapsed over the steering wheel of her Lexus SUV a short time later, according to testimony.
Kennedy has said she swallowed a pill she believed to be her thyroid medication at around 7:30 the morning of the incident, but that she may have mixed up medicine bottles and taken zolpidem, a powerful sleep aid, instead. A blood test confirmed the presence of zolpidem in Kennedy's system, according to trial testimony this week.
"I now know thanks to the tox lab that I must have taken the sleeping medication by mistake," said Kennedy, looking at the jury as she testified.
Kennedy said she made cappuccino, had some carrots, prepared bags for the gym and office and had no problem leaving her apartment and getting to her vehicle the morning of the accident.
Her memory from that morning ends just before she entered the highway, Kennedy said. The next thing she recalls is a knock on the window of her SUV, and a man she thought was a police officer asking if she was OK.
"I was confused by that because I thought I was fine," she said on the stand.
During a contentious cross examination, Kennedy insisted that she would not have stayed behind the wheel if she'd felt the effects of the medication.
"If I'd realized I was impaired, I would have pulled over," she told prosecutor Doreen Lloyd, and also said she doesn't know what the side effects of zolpidem might feel like.
"You've taken this pill for 10 years and you can't tell me whether or not it makes you feel tired after you take it?" Lloyd asked.
"I guess I don't really think about how I'm feeling when I take it," Kennedy replied. "I take it, and then I'm asleep."
The jury has heard testimony from 15 witnesses in the case, and could begin its deliberations Thursday. Defense attorney William Aronwald told CNN Wednesday afternoon that a forensic pharmacologist was the only remaining defense witness.
Sitting in court was Kennedy's mother, Ethel Kennedy, and younger sister Rory, who was a defense witness.
Kennedy said she used zolpidem for travel across time zones but had never taken it in the morning. She testified that she normally kept the 10 mg pills in a sealed plastic bag in the medicine cabinet, away from her thyroid medication, but on July 13, 2012, she had both medications on the vanity because she was packing for a trip.
The night before the accident, Kennedy said on the stand, she'd been to a cocktail party but did not drink because she was driving home.
Kennedy opened her testimony speaking about her family background, saying that she was raised by her mother after her father died when she was 8 years old.
"He was killed when he was running for president," she said.
Under direct examination by her lawyer, Gerald Lefcourt, Kennedy described her human rights work and her healthy lifestyle, saying she visited the gym often and kept an organic garden.
At the start of her defense case Tuesday, Kennedy watched with a smile as Rory Kennedy described their close relationship.
"She's really my best friend," Rory Kennedy on the stand.
When asked about her sister's reputation for truthfulness, she said, "She wrote the book called 'Speak Truth to Power' about her commitment to justice and truth, and that's what she has devoted her life to."
A growing group of supporters has sat in the rows behind the defendant throughout the two trial days, including sister Kathleen Kennedy Townsend, brothers Robert F. Kennedy Jr. and Douglas Kennedy, and their mother, who is 85.
Jurors heard from Bradley Molloy, a drug-detection specialist with New York State Police who evaluated Kennedy after the crash. Some three hours after she allegedly failed three roadside sobriety tests, Kennedy passed those same tests with flying colors, testified Molloy.
Finding no signs of intoxication, Molloy instead began to suspect that Kennedy's car accident and disoriented state had been the result of a medical condition, he said.
Kennedy herself was also worried, and asked for a vial of the blood drawn from her at Northern Westchester Hospital, said Molloy: "She was concerned and she wanted to know what happened to her." Kennedy remained at the hospital overnight for observation.
During opening statements Monday, Assistant District Attorney Stefanie DeNise said that even if Kennedy, 54, had taken the sleep aid unintentionally, she had a responsibility to pull her vehicle off the road safely when she began to feel its effects.
Lefcourt disputed that argument, saying Kennedy had no time to react.
"The zolpidem kicks in, it shuts her down, she's in a state of 'sleep driving,'" he said.
Tuesday, jurors watched another member of Kennedy's defense team spar with an expert witness about the effects of the drug.
Laboratory director Elizabeth Pratt conceded that zolpidem is "a very potent, fast-acting hypnotic," that begins to hamper cognitive ability and motor skills within 15-45 minutes.
Its effect on Kennedy would have peaked at around 9 a.m., testified Pratt, roughly the time Kennedy failed an initial round of sobriety tests by police after a motorist called 911.
Pratt was asked by prosecutor Lloyd whether zolpidem is a "knock-out pill," and replied that the drug would need time to enter the blood stream and brain before it takes full effect.
"And, prior to the time the drug takes effect, does a person still have brain function? Do they still have decision-making ability?" asked Lloyd.
"Yes," replied Pratt.
If convicted, Kennedy could face up to one year in prison.