Almost overnight, a powerful new painkiller has become a $100 million business and a hot Wall Street story.

But nearly as quickly, questions are emerging about how the drug is being sold, and to whom.

The drug, Subsys, is a form of fentanyl, a narcotic that is often used when painkillers like morphine fail to provide relief. The product was approved in 2012 for a relatively small number of people — cancer patients — but has since become an outsize moneymaker for the obscure company that makes it, Insys Therapeutics. In the last year, the company’s sales have soared and its share price has jumped nearly 270 percent.

Behind that business success is an unusual marketing machine that may have pushed Subsys far beyond the use envisioned by the Food and Drug Administration. The F.D.A. approved Subsys only for cancer patients who are already using round-the-clock painkillers, and warned that it should be prescribed only by oncologists and pain specialists. But just 1 percent of prescriptions are written by oncologists, according to data provided by Symphony Health, which analyzes drug trends. About half of the prescriptions were written by pain specialists, and a wide range of doctors prescribed the rest, including general practice physicians, neurologists and even dentists and podiatrists.

Interviews with several former Insys sales representatives suggest the company, based in Chandler, Ariz., has aggressively marketed the painkiller, including to physicians who did not treat many cancer patients and by paying its sales force higher commissions for selling higher doses of the drug.