A sign on Route 23 south of Chillicothe, Ohio, advertises addiction treatment. Opioid addiction and death from overdose is on the rise in Ohio. (Bonnie Jo Mount/The Washington Post)

Pharmaceutical companies don’t make heroin or the other illicit opioids that have caused the majority of U.S. opioid overdose deaths since 2014. But that doesn’t mean they can escape some of the responsibility for the current wave of heroin and fentanyl deaths.

In the 1990s, when the industry began aggressively marketing prescription opioids such as OxyContin, heroin was a minimal presence in American life. There were still surviving older users from the heroin epidemic of the 1960s and 1970s, as well as a smattering of younger users in some cities, but the heroin market was small and in decline.

However, as the number of annual opioid prescriptions in the United States nearly quadrupled beginning in the 1990s to the point that U.S. per capita consumption far exceeded that of any other nation in the world, millions of Americans became addicted to prescription opioids.

As documented by investigative journalists such as Sam Quinones and Pagan Harleman, heroin traffickers sensed a business opportunity: They set up shop in the areas of the United States with the highest prevalence of prescription opioid addiction. Already addicted to legal prescription opioids, it was a short jump for many unfortunate people to switch their allegiance over to cheap, potent, heroin.

The heroin traffickers’ business plan was amoral but wildly successful. About 80 percent of Americans who became heroin-addicted were transfers from prescription opioids, according to an assessment from the National Institutes of Health.

And once heroin markets became reestablished around the country, more people had the opportunity to use heroin without starting with legal opioids first. Thus, even heroin overdoses of people who were never prescribed a legal opioid are part of downstream consequence of opioid manufacturers’ conduct.

Many other parties played a witting or unwitting role in bringing back heroin markets: health-care regulators who didn’t protect the public, doctors who prescribed recklessly and, of course, people who sold and used heroin.

But none of that changes the fact that by working to flood the country with legal prescription opioids, manufacturers established the conditions that allowed illicit opioid markets to return to American communities. Courts that adjudicate the wave of lawsuits drug companies face from addiction-racked cities, counties and states would do well to remember that.