Meta Platforms CEO and billionaire Facebook founder Mark Zuckerberg is set to be questioned for the first time in a US court on Wednesday about Instagram's effect on the mental health of young users, as a landmark trial over youth social media addiction continues.

While Zuckerberg has previously testified on the subject before Congress, the stakes are higher at the jury trial in Los Angeles, California. Meta may have to pay damages if it loses the case, and the verdict could erode Big Tech's longstanding legal defense against claims of user harm.

The lawsuit and others like it are part of a global backlash against social media platforms over children's mental health.

Australia has prohibited access to social media platforms for users under age 16, and other countries, including Spain, are considering similar curbs. In the US, Florida has prohibited companies from allowing users under age 14. Tech industry trade groups are challenging the law in court.

The case involves a California woman who started using Meta's Instagram and Google's YouTube as a child. She alleges the companies sought to profit by hooking kids on their services despite knowing social media could harm their mental health. She alleges the apps fueled her depression and suicidal thoughts and is seeking to hold the companies liable.

Meta and Google have denied the allegations, and pointed to their work to add features that keep users safe. Meta has often pointed to a National Academies of Sciences finding that research does not show social media changes kids' mental health.

The lawsuit serves as a test case for similar claims in a larger group of cases against Meta, Alphabet's Google, Snap and TikTok. Families, school districts and states have filed thousands of lawsuits in the US accusing the companies of fueling a youth mental health crisis.

Zuckerberg is expected to be questioned on Meta's internal studies and discussions of how Instagram use affects younger users.

Over the years, investigative reporting has unearthed internal Meta documents showing the company was aware of potential harm. Meta researchers found that teens who report that Instagram regularly made them feel bad about their bodies saw significantly more “eating disorder adjacent content” than those who did not, Reuters reported in October.

Adam Mosseri, head of Instagram, testified last week that he was unaware of a recent Meta study showing no link between parental supervision and teens' attentiveness to their own social media use. Teens with difficult life circumstances more often said they used Instagram habitually or unintentionally, according to the document shown at trial.

Meta's lawyer told jurors at the trial that the woman's health records show her issues stem from a troubled childhood, and that social media was a creative outlet for her.