Biotech company Elan, Ireland's biggest drugmaker, said 31,800 patients were using its multiplesclerosis treatment Tysabri at the end of June.
Of those currently prescribed the drug, 17,800 are in the US and 13,400 patients are in the remaining 35 countries in which the use of the drug is approved. Approximately 6,600 have been on the drug for at least 18 months.
A further 600 people are taking the drug in global clinical trials.
Elan and its US partner Biogen Idec withdrew Tysabri in February 2005 after two patients developed fatal brain infections.
However, in a statement this morning the companies said there have been "no confirmed cases" of the infection since the product was reintroduced in the US and approved internationally since July 2006.
Davy analyst Jack Gorman said the patient numbers were impressive and supported its full year revenue forecast of $842 million.
The data suggests that 438 patients a week were being prescribed the drug, accelerating from the 392 per week recorded at the end of the first quarter.
Elan will report its second quarter results on Thursday.
Biogen Idec said today its second-quarter profit rose 11 per cent on higher sales of Tysabri, its fastest-growing medicine. The company also raised its 2008 forecast.
Net income climbed to $207 million, or 70 cents a share, from $186.1 million, or 54 cents, a year earlier, company said today in a statement.
Excluding certain costs, Biogen earned 91 cents, beating estimates. Revenue rose 28 per cent to $993.4 million as more doctors prescribed Tysabri.
Biogen has estimated 100,000 patients will be taking Tysabri by 2010.
Additional reporting Bloomberg
