(Adds further details from earnings call)

By Nate Raymond

March 8 (Reuters) - Insys Therapeutics Inc on Thursday posted a bigger fourth-quarter loss than a year ago, hurt by higher costs and soft demand for Subsys, an opioid cancer pain medication at the center of a U.S. investigation.

The Chandler, Arizona-based drugmaker said its net loss widened to $47 million, or 65 cents per share, in the quarter ended Dec. 31, from $3.7 million, or 5 cents per share, a year earlier.

Revenue fell 42.6 percent over that period to $31.5 million, a decline Insys Chief Executive Saeed Motahari attributed to a drop in prescriptions of medications like Subsys containing fentanyl, a synthetic opioid, used to treat pain in cancer patients.

"People are still reluctant to use this product even for appropriate patients that FDA has approved it," Motahari said on a conference call with analysts.

Insys has been embroiled in investigations related to Subsys, an under-the-tongue spray.

Federal prosecutors in Boston have accused seven former executives and managers at Insys, including billionaire founder John Kapoor, of participating in a scheme to bribe doctors to prescribe Subsys and to defraud insurers into paying for it.

They have pleaded not guilty. Several other former Insys employees, including sales representatives, and medical practitioners, have also faced criminal charges.

Following Kapoor's arrest in October, Insys said it expected to pay at least $150 million as part of a settlement with the U.S. Justice Department. No deal has been announced.

Motahari said he had no update on the timing of a settlement, though he was "cautiously optimistic" the company was moving in the right direction.

"We are committed to resolving our outstanding legal issues and look forward to putting those issues behind us as we are radically transforming this company," he said.

Net revenue included $30.7 million from Subsys and $800,000 from Syndros, a liquid dronabinol that Insys launched in mid-2017 to treat anorexia in people with AIDS who have lost weight, and vomiting associated with chemotherapy.

"While I'm not satisfied with our short-term results with either product, there remain opportunities to do better as we move forward," Motahari said.

He said Insys was focused on developing products in the pipeline and was also exploring licensing Subsys and Syndros to companies in Europe and South America. (Reporting by Nate Raymond and Anuron Kumar Mitra; Editing by Sai Sachin Ravikumar and Richard Chang)