Three of the world's largest social media companies will defend themselves in court this week against allegations their platforms deliberately addict children and damage mental health, in the first case of its kind to reach trial.

Meta Platforms, TikTok and YouTube are being sued by a 19-year-old California woman, known as K.G.M., who claims she became addicted to their services from age 10 due to features designed to maximise engagement, according to Reuters. The lawsuit alleges this addiction fuelled depression, self-harm and suicidal thoughts. Snap, which owns Snapchat, settled with the plaintiff last week on undisclosed terms, days before jury selection was due to begin on Tuesday in Los Angeles Superior Court.

The trial represents a critical test of the tech industry's longstanding legal shield. Companies have for years relied on Section 230 of the Communications Decency Act, which protects platforms from liability over user-generated content. However, K.G.M.'s legal team argues the companies should be held accountable for design choices including endless scrolling feeds, frequent notifications and algorithms that recommend connections with strangers.

Meta chief executive Mark Zuckerberg is expected to take the witness stand during proceedings, which are set to last several weeks. Matthew Bergman, the plaintiff's lawyer, told Reuters the case breaks new legal ground. "We are writing on a legal tabula rasa," he said.

According to court documents cited by CNN, K.G.M. accessed the platforms despite her mother's efforts to block them using third-party software. The lawsuit alleges Instagram and TikTok targeted her with content promoting harmful social comparison and negative body image. On Instagram, she was allegedly bullied and subjected to sextortion – where perpetrators threaten to share explicit images unless victims send money or additional photos. The complaint states it took two weeks and coordinated reporting by friends and family before Meta addressed the issue.

The outcome could influence how roughly 1,500 similar personal injury cases against the companies are resolved, with potential damages running into billions of pounds. School districts and state attorneys general have also filed separate lawsuits this year. Sarah Gardner, chief executive of the Heat Initiative, which campaigns for child safety online, told CNN: "These are the tobacco trials of our generation."

The companies have rejected claims their platforms harm young people, pointing to safety features introduced in recent years. Meta spokesperson comments directed CNN to a website stating the lawsuits "misportray our company and the work we do every day to provide young people with safe, valuable experiences online". YouTube spokesperson José Castañeda said the allegations are "simply not true", whilst TikTok did not respond to requests for comment from CNN or Reuters.

According to Reuters, the companies have mounted extensive efforts to shape public opinion, sponsoring parent workshops at schools and partnering with organisations including National PTA and Girl Scouts. They have also retained lawyers who previously represented corporations in major addiction litigation, including cases related to the opioid epidemic.

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