Companies often charge stratospheric prices for drugs for rare diseases — known as orphan drugs — and Acthar’s price is not as high as some. Society generally tolerates those costs to encourage drug companies to develop crucial, possibly lifesaving drugs for these often neglected diseases.

But Questcor did almost no research or development to bring Acthar to market, merely buying the rights to the drug from its previous owner for $100,000 in 2001. And while the manufacturing of Acthar is complex, it accounts for only about 1 cent of every dollar that Questcor charges for the drug.

Moreover, the tiny “orphan” market soon became much bigger. Before long, Questcor began marketing the drug for multiple sclerosis, nephrotic syndrome and rheumatologic conditions, even though there is little evidence that Acthar is more effective for those other conditions than alternatives that are far cheaper. And the company did so without being required to prove that the drug actually works. That is because Acthar was approved for use in 1952, before the Food and Drug Administration required clinical trials to show a drug is effective for a particular disease. Acthar is essentially grandfathered in.

Today, only about 10 percent of the drug’s sales are for infantile spasms. The new uses, Mr. Bailey has told analysts, represent multibillion-dollar opportunities for Acthar and Questcor, its sole maker.

The results have been beyond even the company’s wildest dreams. Sales of Acthar, which accounts for essentially all of Questcor’s sales, totaled nearly $350 million in the first nine months this year, up 145 percent from the period a year earlier. In the same period, Questcor’s earnings per share nearly tripled, to $2.12. In the five years after the big Acthar price increase in August 2007, Questcor shares rose from around 60 cents to about $50, in one of the best performances of any stock in any industry.

But in September, the shares plummeted after Aetna, the big insurer, said it would no longer pay for Acthar, except to treat infantile spasms, because of lack of evidence the drug worked for other diseases. The stock now trades at $26.93.

Peter Wickersham, senior vice president for cost of care at Prime Therapeutics, a pharmacy benefits manager that has found the drug is possibly being overused, says the huge increase in Acthar’s price for patients “just invites the type of scrutiny that it’s received.”