Prosecutors called Hunter Biden's ex-girlfriend Zoe Kestan to the stand, where she testified that she first met him at a Manhattan strip club in late 2017 and dated him for several months afterward.

Kestan bore witness to Hunter Biden's use of crack "every 20 minutes or so."

Kestan, who testified under subpoena and with an immunity agreement with prosecutors, recalled that Hunter Biden's demeanor didn't change after he used crack.

She described him as "cognizant and coherent."

"I remember thinking to myself that there was no change in his behavior," she said. "Nothing had changed, he was the same charming person."

Prosecutors showed jurors photos from Kestan's phone that showed crack pipes strewn about hotel rooms where they stayed.

On cross-examination by defense attorney Abbe Lowell, Hunter Biden's ex-wife Kathleen Buhle testified that she never personally saw Hunter Biden using drugs.

"Did you ever see Hunter using drugs?" Lowell asked.

"No," Buhle replied.

Buhle's answer prompted prosecutors to ask Buhle what she saw "if [she] didn't' see him using drugs?"

"He told me when I found the pipe what it was ... and because he was acting not himself," Buhle said.

Lowell also asked Buhle about Hunter Biden's alcohol use, to which Buhle testified that he abused alcohol for many years.

"He went to rehab the first time in 2003," Buhle said.

Taking the stand as a prosecution witness, Hunter Biden's ex-wife Kathleen Buhle described how, after 25 years, their marriage deteriorated under the weight of his drug and alcohol addiction.

Buhle said she first learned of Hunter Biden's crack addiction when she discovered an empty crack pipe in an ash tray on their side porch the day after their anniversary in 2015. Hunter Biden was kicked out of the Navy in 2014 for testing positive for cocaine, and Buhle said ever since then she was "worried, scared" he would abuse drugs again.

When he was on crack, Buhle testified, "He was not himself … angry, short-tempered, acting in ways he wouldn't when he was sober."

She said she would find drug "remnants in little bags" and drug paraphernalia like "a broken crack pipe" -- often enough that she would search his car before allowing her daughters to use it.

In what could be a blow to one of Abbe Lowell's lines of defense, Buhle said Hunter Biden frequently interacted with friends and family, worked, and socialized while under the influence of crack cocaine -- and would often try to hide it from those close to him.

"Did you see him, for lack of a better word, function?" prosecutor Leo Wise asked.

"Yes," Buhle confirmed.

On redirect examination of FBI Special Agent Erika Jensen, prosecutor David Hines heavily pushed back on the defense's suggestion that no text messages showed Hunter Biden buying drugs during the time he purchased a gun, and their contention that his large cash withdrawals could have been for other purposes.

Hines returned to the key text message Hunter sent while he owned the gun, telling his then-partner Hallie Biden that he was "sleeping on a car" smoking crack.

"You understand what that means, right?" Hines asked the FBI agent.

"Yes," she responded.

"Crack is crack," Hines said.

Hines then returned the issue of Hunter Biden's large cash withdrawals -- over $150,000 in three months -- pushing back on the suggestion from the defense that he used the cash to pay for other things, such as Airbnb rentals.

Hines displayed bank records that showed Hunter paid for Airbnb rentals with a Visa check card.

Hines also pointed out that when Hunter Biden bought alcohol, he did so with a credit card instead of cash -- making the point that drug dealers generally favor cash transactions.

Hallie Biden, Hunter Biden's former romantic partner who is the widow of his brother Beau Biden, has taken the stand as the prosecution's next witness.

Prosecutors have said they expect her to testify about Hunter Biden's drug at the time of his gun purchase in October 2018.

Before gun store clerk Gordon Cleveland stepped off the stand, he testified that Hunter Biden did not exhibit any symptoms of drug or alcohol abuse when he purchased the gun at the center of the case.

When asked by defense attorney Abbe Lowell if Hunter Biden exhibited any symptoms at the time of his gun purchase, such as being "glassy eyed" or smelling of alcohol, Cleveland replied, "None at all."