The trial against four pharmacy chains over their role in the opioid crisis began on Monday, the Associated Press reports.

CVS, Giant Eagle, Walgreens and Walmart are being sued in a federal court in Cleveland. The trial involves two Ohio counties, Trumbull County and Lake County. The case is considered a “bellwether” test case. The pharmacy chains face more than 3,300 similar lawsuits filed by state and local governments nationwide, the article notes.

The trial will focus on the harm to the two counties and the pharmacy chains’ response. The companies have argued in court filings that their pharmacists were filling prescriptions written by doctors for legitimate medical purposes.

Lawyers for the two counties say 80 million prescription opioids were dispensed in Trumbull County between 2012 and 2016, and 61 million prescription opioids were dispensed in Lake County.

The pharmacy companies argue they followed guidelines established by the Drug Enforcement Administration and the state of Ohio in dispensing opioids.