On the day the city of Baltimore announced an $80 million settlement with the pharmacy chain Walgreens, a doctor in Boston admitted to improperly prescribing opioids to a patient who had to be hospitalized for a medication overdose.

Edgar Ross, a prominent pain specialist, agreed to pay $25,000 and work with a compliance monitor to resolve allegations brought by the Drug Enforcement Administration and federal prosecutors in Massachusetts.

He was also an expert witness for Walgreens in the Baltimore lawsuit.

As the city’s opioid trial has proceeded against the two defendants who didn’t settle — the drug distributors McKesson and AmerisourceBergen — it has become, in many ways, a contest of expert witnesses.

Both sides have offered experts to testify on the underlying causes of the opioid crisis, the number of painkillers that should have been sent to a city the size of Baltimore, and how many shipments of oxycodone should have been blocked before they reached local pharmacies.

These experts are highly qualified — and highly paid — to give their opinions, and say their conclusions aren’t based on who signed their checks. Their testimony has taken up much of the six-week opioid trial, offering a glimpse into the little-known industry that connects lawyers with expert witnesses in virtually any subject.

“The world of expert witnesses is a big business,” said Michele Gilman, the director of the Saul Ewing Civil Advocacy Clinic at the University of Baltimore School of Law.

“There’s a sense in the legal profession, and probably the public as well, that you can, with a deep pocket, find an expert willing to testify in a way that supports your theory of the case,” she said.

Expert testimony has proliferated in the last two decades and is allowed if a judge agrees it will help the “trier of fact” (usually a jury) understand the evidence in a case, said Steven Silverman, a Baltimore attorney. Lawyers increasingly use experts to cover their bases in case of an appeal.

“The more complex the issues, the more the parties feel compelled to support their opinions with expert testimony,” he said. “Especially in litigation like this, where potentially hundreds of millions if not billions of dollars are on the line, they’re not going to take any chances. You can’t rely on common sense.”

Ross, the Walgreens expert, is an unusual example because of the DEA settlement. He would not have been called to testify at Baltimore’s opioid trial after Walgreens settled, and it’s not clear if his legal troubles contributed to Walgreens’ decision to exit the case with an $80 million payment to the city. Walgreens declined to comment.

Ross, whose lawyer did not respond to a request for comment, admitted as part of the DEA settlement that he prescribed oxycodone and other drugs to a longtime patient even after she admitted to taking more medication than prescribed and sought early refills. She eventually overdosed and was admitted to the hospital in 2018.

Brigham and Women’s Hospital, where Ross was a senior clinician and former director of the Pain Management Center, eventually arranged for another doctor to treat the patient, according to the agreement.

In an email, a spokesperson for the health system thanked the U.S. Attorney’s Office in Massachusetts for reviewing the matter and said Ross retired in 2023.

Ross received $750 per hour for his work as an expert witness for Walgreens, according to his report. His 62-page report described how opioid prescribing increased after medical guidelines changed in the 1990s and early 2000s — due in part to an intense marketing campaign by Purdue Pharma, the maker of OxyContin — and how medical boards, including in Maryland, encouraged doctors to prescribe more opioids to address chronic pain.

He also opined that it is not typical for a person who is prescribed opioids to become addicted and later turn to heroin or other illicit drugs when prescription painkillers are no longer accessible. (Conversely, other experts say that more than 80% of people with opioid use disorder started using prescription painkillers before transitioning to heroin.)

As part of his DEA settlement agreement, however, Ross agreed that he prescribed his patient a cocktail of drugs “without always appropriately documenting the risks.”

“Dr. Ross warned this patient about the risks associated with the amount of opioids they were taking, but Dr. Ross nevertheless continued prescribing these drugs,” according to the agreement.

One of McKesson’s experts, Dr. Christopher Gilligan, was previously the associate chief medical officer at Brigham and Women’s Hospital. Gilligan reached similar conclusions as Ross about the causes of the opioid crisis and how people with opioid use disorder transition to using heroin.

“The standard of care for prescribing opioids is a standard for doctors, the ones who prescribe opioids and have the training to decide whether to prescribe opioids,” Gilligan testified. “Distributors, quite frankly, don’t have anything to do with that decision. They don’t treat patients.”

Gilligan has testified for McKesson in several other cases, he said on the stand. He was paid $800 an hour for his work.

Unsurprisingly, Baltimore’s expert witnesses have supported the city’s version of the case. They included a longtime DEA employee, a professor of health policy and an economist, each billing between $500 and $700 per hour for their work.

Ruth Carter, a former DEA manager who was a central witness for the city, has earned nearly $3 million testifying for plaintiffs in opioid litigation, according to her testimony.

McKesson and AmerisourceBergen brought in their own experts to challenge city witnesses’ conclusions and methodologies in detailed back-and-forth testimony about statistics that bordered on tedious.

Both sides traded barbs about expert witnesses in their closing arguments Thursday. Bill Carmody, the city’s lead attorney, criticized the drug companies for relying primarily on expert witnesses for their defense.

The drug companies shot back: “These experts were paid a lot of money to testify, but you get to be the ones to decide whether that testimony has any value,” Robert Nicholas, a lawyer for AmerisourceBergen, told jurors.

This sort of back-and-forth is typical, especially in a big, complicated case like the opioid trial.

“We have an adversarial system where each side is zealously advocating for their client,” Gilman said. “They’re going to find an expert that supports their theory of the case.”

But the cost of expert witnesses is a source of concern. A regular person can’t necessarily pay $600 an hour for an expert to testify in their medical malpractice case, for example.

“Litigation is for corporations and independently wealthy people, and that’s it,” Silverman said. “The average person has no access to civil justice in this country, and the only caveat to that is if a case has got a potential value high enough that a law firm wants to … do the work on contingency.”

Baltimore’s outside counsel in the opioid trial, the law firm Susman Godfrey, is working on contingency. That means the firm only takes home money if it wins settlements or verdicts for the city.

Baltimore has already won more money in settlements from opioid defendants than the entire state of Maryland received as part of a group deal with drug distributors.

The city’s lawsuit, like other opioid litigation across the country, accuses the drug companies of ignoring their legal obligation to monitor and block suspicious orders of opioids before they were shipped to Baltimore-area pharmacies. The millions of painkillers sent to Baltimore far exceeded what was medically necessary, the city claims, and contributed to what became one of the deadliest opioid epidemics in America.

The complexity of the issues involved can pose a problem, too, Gilman said. Jurors might be inclined to agree with the expert who has a more appealing presentation style if they don’t understand the testimony, which in this trial has included the details of statistical regressions and algorithms designed to identify suspiciously large orders of opioids.

Judges decide whether a witness has sufficient “knowledge, skill, experience, training, or education” in their field to testify as an expert. The Maryland courts recently adopted a new standard for assessing expert testimony.

But judges are not experts in those fields, which can make it challenging for them to evaluate a purported expert’s claims, Gilman said. That’s especially concerning in criminal courts, where some fields of forensic science have been debunked or seriously questioned after experts used them to support convictions.

Experts “are clothed with so much authority in the courtroom that they can become conduits for junk science,” Gilman said.

“There’s pressure on trial court judges to serve as a gatekeeper and make sure that junk science doesn’t get before jurors, but judges went to law school,” she said. “The gatekeeper doesn’t necessarily have the background.”