Chugai Pharmaceutical Co. (4519.TO) said Wednesday that it is undeniable that its rheumatoid arthritis drug Actemra might be a causal factor in the deaths of 15 patients.

According to research of 4,915 cases conducted by the doctors who prescribed the drug, heavy side effects were found in 221 cases, and 15 deaths were undeniably connected to the drug during the 10 months to February, a Chugai spokesman said.

The spokesman said that the company will provide more information so that doctors can have a better understanding of the drug's side effects.

Actemra was approved in April in Japan last year, but in order to verify side effects undetected by clinical tests, the Health Ministry requested an investigation of all patients who have taken it.

The drug is also currently under regulatory review by the U.S. Food and Drug Administration.

-By Yoshio Takahashi, Dow Jones Newswires; 813-5255-2929; yoshio.takahashi@dowjones.com