Daiichi Sankyo Co. risks losing a slice of revenue to generic-medicine competition next year. Now, the drugmaker's plans to fill the gap have hit a roadblock and investors worry that growth may flounder.

The Tokyo-based company's best-selling medicine is set to lose patent protection in 2016, and a new blood thinner had been seen as one of its best chances to offset the hit. Instead, a U.S. regulatory warning placed on the new drug's packaging may limit its market potential.

Finding a way to boost earnings is becoming urgent for Daiichi Sankyo, once a high flyer with a lineup of promising drugs. Its 2008 purchase of India's Ranbaxy Laboratories Ltd. resulted in share declines and lost profits. Benicar, the hypertension treatment whose patent is slated to expire next year, brings in about 27 percent of annual revenue.