For the second time in less than four weeks, the US Food and Drug Administration (FDA) has asked the world's largest drug maker Pfizer Inc. to stop conducting patient tests of an experimental drug.

The latest FDA directive relates to stoppage of testing of tanezumab in patients suffering from chronic lower back pain and nerve damage caused by diabetes. This is the second such directive from Federal regulators who halted testing of the drug in osteoarthritis patients.

Both these tests were in late stages and with the FDA seeking a halt to the tests, Pfizer will not be able to add new patients to the sample size. In addition, those already participating will have to stop taking the pills.

The latest move by the FDA is said to be the result of complaints that patients being tested with the new pills were showing signs of worsening osteoarthritis that required joint replacements.

In the last 18 months, Pfizer Inc. has witnessed failure of as many as a dozen studies that included tests for Alzheimer’s disease, cancer and back pain. Poor efficacy or safety related problems were ascribed as the main reason for these failures.

The drug maker, which spends more than $9 billion each year in research, had pinned great hopes on the drug for Alzheimer's disease with doctors too expecting it to be a path breaker to stop or reverse mind-robbing ailments.

The company has reported that three other non-arthritis studies will continue as per schedule. The other studies relate to cancer pain and chronic pancreatitis.