Johnson & Johnson, one of the world's largest drug manufacturers, has gone on trial in a multi-billion dollar lawsuit by the US state of Oklahoma.
Prosecutors accuse the firm of deceptively marketing painkillers and downplaying addiction risks, fuelling a so-called "opioid epidemic".
Johnson & Johnson denies wrongdoing and says it marketed products responsibly.
It is the first of 2,000 cases brought by state, local and tribal governments against pharmaceutical firms in the US.
On average, 130 Americans die from an opioid overdose every day, according to the Centers for Disease Control and Prevention.
In 2017, of the 70,200 people who died from overdose, 68% involved a prescription or illegal opioid.
Johnson & Johnson produces a fentanyl patch which can be prescribed for severe pain.
Fentanyl belongs to a class of drugs known as opioid analgesics, which change how the body feels and responds to pain. It is also used as a recreational drug, often mixed with heroin and cocaine.
In its court filing, Oklahoma alleged that Johnson & Johnson was the "kingpin" behind "the worst man-made public health crisis in [the] state's history," growing and importing raw materials which other drug-makers used for their own products.
The state argues that Johnson & Johnson created a public nuisance which will cost between $12.7bn (£10.02bn) and $17.5bn (£13.8bn) to remedy over the next 20 to 30 years.
But Johnson & Johnson argues that the public nuisance law does not apply in this instance.
This is the latest in a string of legal cases against painkiller manufacturers over prescription drugs.
Earlier this month, Israel-based Teva Pharmaceuticals agreed to an $85m (£67m) settlement with Oklahoma over a similar lawsuit which claimed its opioids had contributed to the deaths of thousands of people.
Drugmaker Purdue Pharma - which produces the prescription painkiller OxyContin - also reached a $270m settlement with Oklahoma in a separate case. The wealthy Sackler family, which owns the firm, has been charged with fraudulently transferring money from the company to protect its funds from litigation.
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