Subsys is a form of fentanyl, a narcotic that is often used when other painkillers fail to provide relief. The product, which is sprayed under the tongue, begins working in as little as five minutes, which is useful for patients who experience sharp bursts of pain. But it carries a high risk of dependency and can cause respiratory distress — and death — for those who are not using painkillers regularly.

Image Insys Therapeutics, manufacturer of Subsys, a powerful and addictive narcotic, made large payments to pain specialists who recently faced legal or disciplinary action.

Several of the highest-paid doctors said they spoke on behalf of Subsys because they believed in the product.

“I’m not a big ‘talk for companies’ kind of guy,” said Dr. Somerville, who noted that he was a former president of the Texas Pain Society. “They liked my personality and how I approached things, and I liked their product.”

Dr. Somerville, who was a top prescriber of Subsys, said Insys representatives never questioned him about the Texas Medical Board’s ruling, and he continued speaking about Subsys for a few months after the board’s decision. Insys officials said they removed him as a speaker after learning of the disciplinary action.

Dr. Somerville took issue with the medical board’s claims, saying the board did not have evidence that he contributed to the patients’ deaths and that the action was the result of a vendetta against doctors who frequently prescribe opioids. “I’m very aggressive in trying to help my patients,” he said. “I’m a very caring doctor. I like money, but that’s not my god, O.K.?”

Other top speakers for Insys have also faced regulatory or legal trouble. Dr. Paul Madison, a pain specialist in Chicago, was indicted in 2012 on federal fraud charges after the authorities claimed that the surgical center he owned billed insurers for procedures that were never performed. Insys paid Dr. Madison nearly $33,000 in 2013. Another top Insys speaker, Dr. Steven Y. Chun of Sarasota, Fla., paid $750,000 in February to settle federal charges that he had billed for procedures he did not perform. Dr. Chun received $45,000 from Insys in 2013, according to the database.

Dr. Chun said that he admitted no wrongdoing in the federal settlement and that he provided excellent care to his patients. Dr. Madison said he had done nothing improper; both doctors declined to comment further.