Conrad Roy was 18 when he died in July 2014 of carbon monoxide poisoning after locking himself in his truck.
Legal experts are watching the trial closely because it could set a legal precedent on whether it is a crime to tell someone to commit suicide.
The prosecution rested its case Thursday.
Prosecutors allege Carter acted recklessly when she pressured Roy into suicide.
"She's following his lead into a very dark place", Breggin said. Dr. Peter Breggin testified that because of a recent change in her antidepressant medication, Carter was not in her right mind and "was unable to form intent" when Roy took his own life.
Defense Attorney Cory Madera makes notes as defendant Michelle Carter listens with attorney Joseph Cataldo (not shown) during her trial at Taunton Juvenile Court in Taunton, Mass., Monday, June 12, 2017.
She began taking a new drug, Celexa, in April 2014 for "impulse control issues" and had become "involuntarily intoxicated" by the medication. He also said Roy researched what type of medicine would kill him in his sleep.
Michelle Carter appears in court on Monday, May 5, 2017.
Such drugs can impair judgment, wisdom, understanding, love and empathy, he said - especially in the adolescent brain, which is still developing and is "more susceptible to harm and all intrusions".
Breggin spoke directly to Bristol County Juvenile Court Judge Lawrence Moniz about his theories regarding what was going on in Michelle Carter's brain at the time of the texting-related suicide, confirming that he had reviewed Carter's text messages, police records, and spoke to several people close to her. Judge Moniz will independently determine whether or not Michelle Carter is innocent or guilty at the conclusion of her trial, as the 20-year-old opted for a bench trial rather than a jury trial. Roy was reportedly plagued by visions of the devil, an image that haunted Carter's nightmares.
The prosecution, however, bolstered their case by revealing Carter was on the phone with Roy for 46 minutes as he took his own life.
"My life's an abortion". And this boy that she loves now says in the face of her great distress that he is going to kill himself and its going to happen tonight and he even has plans. "My negative thoughts have controlled me to the point where I'm legit going insane".
On July 15, 2014, Rayburn said Carter met with a therapist, who did not indicate that Carter had any symptoms of involuntary intoxication.
In his expert opinion, Dr. Breggin told the court that Michelle Carter would do "anything to help Roy".
"I would be surprised if she had told him about it", Breggin said, maintaining that he believes Carter did cut herself.
Carter wrote that she "told him to get back in".
"I'm trying my best to dig you out", she allegedly wrote in one text message. Carter is charged with involuntary manslaughter for encouraging Conrad Roy III to kill himself in July 2014. She has been labeled a "youthful offender" by prosecutors, a distinction that could result in a long prison sentence if she is found guilty.