The young woman at the heart of the landmark trial about the addictive nature of social media testified for the first time on Thursday, saying she got hooked on YouTube starting at age six and Instagram at nine. By the time she was 10, she said, she had become depressed and was engaging in self-harm.

The woman, who is now 20 and known by her initials KGM, is the lead plaintiff in an expansive lawsuit against YouTube and Meta, which owns Instagram and Facebook. The crux of the case alleges social media companies intentionally create addictive products, leading to mental health issues in young people.

KGM testified on Thursday that her use of social media made her anxious and insecure, and features like beauty filters distorted her self-image. But when her mother took away her cellphone, KGM said she would become filled with panic that she was missing out on something. She said she still uses social media today.

“I can’t, it’s too hard to be without it,” she said when questioned by her lawyer in Los Angeles county superior court, according to Reuters.

The cycle of social media use caused KGM to have strained relationships with her family and in school, she said. She said she had suicidal thoughts and began cutting herself as a “coping mechanism to deal with my depression”.

The trial is the first in a consolidated group of cases brought against Meta, TikTok, YouTube and Snap on behalf of more than 1,600 plaintiffs, including more than 350 families and 250 school districts. KGM’s case is also the first of more than 20 “bellwether” trials, which are used to gauge juries’ reactions and potential verdicts, as well as set legal precedent. If jurors rule in favor of the plaintiffs, the social media companies could face harsh financial penalties and mandates to change the way their platforms function.

KGM’s lawyers allege that some of the features these social media companies built into their platforms, such as infinite scroll and video autoplay, are designed to keep people on the apps and add to their addictive quality. The lawyers also allege that “like” buttons feed into teens’ desire for validation. Their legal arguments mirror those brought against big tobacco in the 1990s.

TikTok and Snap were originally named as defendants in KGM’s case, but just before the trial began last month, both companies reached settlement agreements, the terms of which have not been publicly disclosed.

Meta and YouTube deny wrongdoing in the case. A YouTube spokesperson, José Castañeda, called the allegations in the lawsuits “simply not true” and said that providing young people with a “safer, healthier experience has always been core to our work”.

Meta filed a brief on Wednesday arguing that KGM’s mental health issues were exacerbated by a difficult home life. “The evidence will show she faced many significant, difficult challenges well before she ever used social media,” said Liza Crenshaw, a Meta spokesperson.

KGM, who now works as a personal shopper at Walmart, testified on Thursday that her mother had occasionally been abusive and hit her, according to Reuters. But she also testified that her mother was a loving parent who was working hard to raise three children. KGM said she still lives with her mother.

In the coming days, jurors are expected to hear from KGM’s mother, Karen, and a child and adolescent psychiatrist, Kara Bagot.

KGM’s former therapist, Victoria Burke, testified on Wednesday, saying she worked with KGM over several months when she was 13 and 14 and diagnosed her with body dysmorphic disorder and social phobia, according to NBC News. The therapist said the teen would often turn to her phone to make it look as if she were “doing something rather than sitting and being perceived as having no friends”.

Burke said she believes social media was a “contributing factor” in KGM’s mental health issues.

Mark Zuckerberg, the chief executive of Meta; Adam Mosseri, the head of Instagram; and Cristos Goodrow, the vice-president of engineering at YouTube, also testified over the past two weeks. The trial, which began in late January, is expected to continue for another two to four weeks.