The Shanghai-based manufacturer of a leukemia drug that was banned by China's drug watchdog after some patients reported leg pains and difficulty walking said Monday it was investigating the problem.

The State Food and Drug Administration said over the weekend it had suspended the sale of methotrexate made by Shanghai Hualian Pharmaceutical Co. Ltd. The drug is widely used to treat leukemia and other cancers, as well as autoimmune diseases such as rheumatoid arthritis.

"We are responsible for the patients and we take this issue very seriously," said Yin Qinxie, spokesman for Shanghai Pharmaceutical (Group) Co., China's biggest drug maker.

Yin said the company received reports on July 6 that some patients who received doses of methotrexate made by its unit Shanghai Hualian Pharmaceutical Co. had felt leg pains and experienced difficulty walking.

"Experts are discussing and investigating it," Yin said. "We started trying to figure out the problem as soon as we got the reports on bad reactions," he said.

The Chinese government has been trying to toughen its drug regulation amid mounting criticism at home and aboard that the quality of its drugs, food and other products is poorly regulated.

In the past week alone, a former department head at the State Food and Drug Administration was sentenced to death on bribery charges, and it was announced that authorities had withdrawn the production licenses of five drug makers over the last year and penalized 128 others.

China's pharmaceutical industry is lucrative but poorly regulated. Some companies try to cash in by substituting fake or substandard ingredients.

The country is currently overhauling its chaotic food and drug safety mechanisms, which are handicapped by competition between government agencies, murky laws and corruption.