Oct 14 (Reuters) - U.S. Food and Drug Administration staff recommended that Pfizer Inc keep a severe warning on its controversial quit-smoking drug Chantix.

Approved in 2006, Chantix has been one of Pfizer's most controversial drugs, after severe side effects including suicidal thoughts, erratic behavior and drowsiness came to light in 2007.

The FDA placed a black box warning on the product in 2009, warning users of the neuropsychological effects. A black box warning is the most severe and most restrictive kind of warning on a product.

The FDA staff said on Tuesday observational post-marketing studies submitted by the company "had a number of study design limitations." (http://1.usa.gov/1CgJYlM)

The document said the studies did not cover the full range of the neuropsychiatric adverse events seen in postmarketing adverse event reports associated with varenicline, which is sold as Chantix.

"These limitations may underestimate the actual incidence of neuropsychiatric adverse events...associated with varenicline," the FDA said.

A panel of FDA staff and experts will vote on the recommendations on Oct. 16.

(Reporting by Vidya L Nathan and Amrutha Penumudi in Bangalore; Editing by Don Sebastian)