Pfizer (NYSE:PFE) has stopped international shipments of the smoking-cessation drug Chantix (varenicline) after concluding it had elevated levels of a potential carcinogen. The substance in question, nitrosamines, are organic compounds thought to cause cancer in various organisms. “We believe the benefits of Chantix/Champix outweigh the very low potential risks, if any, posed by nitrosamine exposure…