Pfizer is being schooled in the challenges of sterile drug manufacturing that come with its $15 billion buyout of Hospira. The New York drug giant has temporarily suspended production at a plant in India that the FDA cited three years ago with a warning letter after a new visit found ongoing problems. Pfizer ($PFE) in an email today confirmed that production was put on hold while the observations were being evaluated. It said the company has formally responded to the FDA observations.