A Philadelphia law firm has filed a proposed class-action lawsuit against a number of opioid manufacturers, alleging that they are responsible for the medical needs of babies exposed to opioids before birth.

John Weston, the attorney at Sacks Weston Diamond who brought the suit Friday on behalf of an anonymous baby boy and his mother, said he believes the case is the first of its kind in Pennsylvania. Lawsuits on behalf of babies with neonatal abstinence syndrome, or NAS, have been filed in several other states, but are not as common as the wave of litigation against pharmaceutical companies, often filed by state and local governments seeking to recover costs related to the opioid epidemic.

"It's basically the same theory as all of the other opioid cases," which allege that pharmaceutical companies sparked the opioid crisis by aggressively marketing opioid pain medications and downplaying their addictive properties, Weston said. "It's just a different group of plaintiffs" — children exposed to opioids in the womb who display symptoms of withdrawal at birth.

"You can argue about who's at fault," he said, "but it's certainly not the kid."

If the suit is successful, Weston said, the class of plaintiffs who stand to benefit would be limited to children whose mothers could prove their own addiction began with prescription opioids.

Though the suit claims that newborns with NAS face long-term adverse health outcomes, questions about lasting harm are far from settled because unbiased studies are hard to do. While the symptoms of newborn withdrawal — such as excessive crying, fever, tremors — go away within days or weeks, some studies suggest opioids can leave children with behavioral, cognitive, vision, and movement problems.

But researchers say the effects of licit and illicit opioids are hard to isolate from the effects of other substances mothers might have used, such as alcohol, and from the impact of factors such as poverty and poor prenatal care.

Another obstacle to holding companies liable for children's long-term problems is that many women trying to control their addiction are prescribed milder opioids, either methadone or buprenorphine, that cut cravings without delivering euphoric effects. Abruptly quitting opioids increases the risk of miscarriage, so pregnant women are urged to continue or begin such medication-assisted treatment.

Pennsylvania Medicaid data consistently show that the majority of pregnant women taking prescribed opioids are on buprenorphine or methadone. In Tennessee, which collects data on the sources of neonatal drug exposure, 70 percent of cases were linked to the mothers' medication-assisted treatment in 2017.