Michelle Carter, who has been charged with involuntary manslaughter in the death of 18-year-old Conrad Roy, was taking antidepressants during the time when she texted her boyfriend ways to commit suicide, a medical expert for the defense testified Monday. Dr. Peter Breggin described the Massachusetts woman as a “very troubled youngster,” according to WCVB-TV.

"The texting, particularly with her friends, also shows a side which wasn't apparent anywhere else — which is that of a very troubled youngster underneath," Breggin said, adding that Carter took antidepressants like Celexa, which caused her to be involuntarily intoxicated.

Read: How Michelle Carter Urged Boyfriend To Commit Suicide

"You're now telling us that she didn't know right from wrong at that same time," prosecutor Katie Rayburn questioned Breggin during cross-examination, to which the psychiatrist replied: "Well actually, she thought she was doing absolutely the right thing."

Roy committed suicide in the summer of 2014 at the age of 18 by using carbon monoxide from a truck’s exhaust system to poison himself. Prosecutors say Carter encouraged Roy to kill himself and told him through text messages to use a generator and carbon dioxide during the suicide attempt.

In the opening statements of the trial, prosecutors argued that Carter wanted to be the “grieving girlfriend.”

“The defendant needed something to get their attention,” Assistant District Attorney Maryclare Flynn said. “She used Conrad as a pawn in her sick game of life and death.”

However, the defense argued Roy was already suicidal, and Carter was only trying to help him.

Steven Verronneau revealed that Roy had googled “suicide by cop” and visited a website explaining “easy, quick and painless ways to commit suicide.” He also said Roy researched what type of medicine would kill him in his sleep.

During Monday's trial, Breggin said he reviewed online messages between Roy and Carter, in which Roy convinced Carter into supporting his wishes to kill himself.

Last week's hearings saw prosecutors present numerous jarring text message conversations between Carter and Roy in which Carter appeared to repeatedly encourage Roy to take his own life despite his apparent hesitation.

“I guess [that I’m frustrated], just because you always say you are gonna do it but you don’t, but last night I know you really wanted to do it and I’m not mad. Well, I mean kind of, I guess, just because you always say you’re gonna do it… but you don’t, but last night I knew you really wanted to and I’m not mad,” Carter texted Roy.

Read: Text Messages Teen Sent Encouraging Boyfriend To Kill Himself

Flynn argued during last week's hearing that Roy and Carter were on the phone as he was committing suicide and at one point, he attempted to get out of his truck.

“Conrad got out of his truck as he was being poisoned and he got scared,” Flynn said. “The defendant [expletive] told him to get back in.”

During Monday's trial, the prosecution introduced YouTube videos depicting Roy discussing his depression and social anxiety as evidence. However, defense attorneys for Carter said the videos implied Roy was already suicidal.