U.S. regulators have approved Shire Pharmaceuticals Group Plc’s top-selling children’s hyperactivity treatment, Adderall XR, for use by adults, Britain’s third-biggest drugmaker said on Thursday.

Shire said Adderall XR, approved to treat children in the United States since November 2001, had been cleared to treat adults with attention deficit hyperactivity disorder (ADHD), a condition marked by the reduced ability to concentrate, difficulty in organising and impulsive behaviour.

Analysts said the move could add about $50-$100 million to Adderall XR sales next year. But some were disappointed the firm had not won immediate approval for higher doses of 40, 50 and 60 milligrams.

“It does not give them quite such a neat package to take to the market,” said Paul Diggle, an analyst at Code Securities.

A Shire spokeswoman said the U.S. Food and Drug Administration (FDA) wanted further evidence that higher doses of Adderall XR were more effective than lower doses. She did not say when the firm would submit the required data.

Some studies have shown that up to 65 percent of children with ADHD continue to exhibit symptoms into adulthood, and Shire estimates that over 8 million adults in the United States may show symptoms of the illness but that only 600,000 are treated.

“This is a large potential market and adds to the growth prospects for this compound,” Numis Securities analyst Robin Gilbert wrote in a research note to clients.

But analysts also noted that an estimated 15-20 percent of Adderall XR prescriptions were already being written for adults and that the drug would face competition from Eli Lilly & Co’s Strattera, which is already approved to treat adults.