lawyer for several large California communities accused four drugmakers of causing a deadly wave of opioid addiction with their aggressive marketing of pain pills, while defense attorneys said the firms followed the law, on the opening day of a trial closely watched by the pharmaceutical industry.

“Without an avalanche of prescription opioids, there wouldn’t be an opioid epidemic,” said Fidelma Fitzpatrick, a plaintiffs’ lawyer hired to represent the counties of Los Angeles, Orange and Santa Clara and the city of Oakland.

The monthslong trial, being held fully by videoconference and decided solely by a judge, began Monday with opening remarks from Ms. Fitzpatrick and attorneys for the drugmakers being accused of misconduct— Johnson & Johnson, JNJ +0.71% Teva Pharmaceutical Ltd. TEVA -1.28% , Endo International ENDP -2.27% PLC and AbbVie Inc.’s Allergan.

The drugmakers argued Monday that they properly marketed their drugs under regulatory oversight and that the plaintiffs won’t be able to show examples of local deaths that stemmed from their prescriptions.

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