Vivus said prescriptions for obesity drug Qsymia, its main product, rose nearly 68% in the month ended Dec. 21 compared with the preceding four weeks, well above industry forecasts.

The drugmaker said it shipped 12,978 prescriptions of Qsymia in the latest period, up from 7,749 in the four weeks ended Nov. 23.

New Qsymia prescriptions totaled 5,229 in the latest period.

Healthcare information provider IMS Health had estimated 1,327 new prescriptions, according to Cowen and Co analyst Simos Simeonidis, who expects the drug to generate annual U.S. sales of $1 billion by 2019.

Initial sales of Qsymia were seen as disappointing, prompting one of Vivus's top shareholders to call for the sale of the company. Vivus blamed a lack of insurance reimbursements for the low sales.

Qsymia, approved in July, was the first new weight-loss drug to hit the market in more than a decade.

With some two-thirds of Americans considered overweight or obese, Qsymia and Arena Pharmaceuticals's Belviq, which has also been approved as a weight-loss drug, comes with high expectations.