The shares of Pfizer fell by about 4% following the reports of four deaths in a clinical trial of the company. The clinical trials were conducted to evaluate tofacitinib as a treatment for rheumatoid arthritis. The incident was related to the phase III trials of the company involving 792 patients. These patients were given a 10mg dose of the drug.

The new of death came just one week after Pfizer announced positive results from study showing tofacitinib met all its primary requirements at the 10mg dose. Results of the first of these late-stage trials show that tofacitinib induced significant response rates and improved physical functionality in patients with the disease.

An investigation was launched in the matter of deaths but Pfizer said that three of these deaths were not related to its drug. According to Pfizer, only one case, in which a patient died of respiratory failure, was related to its drug.

The company is expecting that the success of tofacitinib will help it in overcoming the loss. Analysts believe that if the drug will be success in the market, it could generate sales of $2-$3 billion.