Image copyright WCVB Image caption Michelle Carter, 20, is being tried in juvenile court

A 20-year-old US woman on trial for urging her suicidal boyfriend to kill himself was "involuntarily intoxicated" by anti-depressants at the time, a psychiatrist says.

Michelle Carter had switched to a new drug that left her "unable to form intent", Dr Peter Breggin said.

She is charged with involuntary manslaughter for her role in the suicide of Conrad Roy III.

But her lawyers say she had initially tried to make Roy seek help.

Conrad Roy was found dead from carbon monoxide poisoning in his vehicle at a Kmart car park in Fairhaven, Massachusetts, on 13 July 2014.

Image copyright WCVB Image caption Mr Roy had been due to attend university in the autumn

Dr Breggin said he had reviewed texts and Facebook messages between Mr Roy, then 18, and Ms Carter, then 17, and described her as overwhelmed by her boyfriend's obsession with suicide.

But he said the switch in medication from Prozac to Celexa made her believe that she could help Mr Roy get what he wanted.

Celexa is one of a group of anti-depressants known as SSRIs and targets the brain's frontal lobe, which affects empathy and decision-making, Dr Breggin said.

Ms Carter found herself in the grip of a "grandiose" delusion that she could help her boyfriend get to heaven, he said, the Boston Globe reported.

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She sent Mr Roy messages such as "You need to do it, Conrad" and "All you have to do is turn the generator on and you will be free and happy".

In another message she wrote: "You're finally going to be happy in heaven. No more pain. It's okay to be scared and it's normal. I mean, you're about to die."

But she did so because she thought she had "found a way to help her boyfriend", Dr Breggin said.

"She's not thinking she's doing something criminal," he said.

Ms Carter continued to text Mr Roy after his death, including one message saying that she had raised $2,500 in his memory.

Image copyright Facebook Image caption Michelle Carter reportedly texted Conrad Roy that his parents would "get over" his suicide

Prosecutors allege Ms Carter drove the Massachusetts 18-year-old to his death to attract sympathy.

Texts show he wavered in his plan to follow through with the suicide, at one point getting out of his pick-up truck.

The court heard that Ms Carter, who was then 17, replied: "Get the f*** back in the car."

Ms Carter has asked a judge to rule on the case, rather than a jury of her peers.

Lawyers had requested the case be dismissed on the grounds of right to free speech.

But a juvenile court judge ruled that encouraging suicide was not protected under the US constitution.

Where to get help

If you are depressed and need to ask for help, there's advice on who to contact at BBC Advice.

From Canada or US: If you're in an emergency, please call 911. If you or someone you know is suffering with mental-health issues, call Kids Help Phone at 1-800-668-6868. If you're in the US, you can text HOME to 741741

From UK: Call Samaritans on 116123 or Childline on 0800 1111