Scientists at Pfizer are reevaluating the popular smoking-cessation drug Chantix (varenicline) in response to reports of negative side effects, Scientific American reported Aug. 27.

The Institute for Safe Medication Practices has documented 988 "adverse events" relating to the drug -- including acute psychiatric episodes such as seizures, psychosis and suicidal depression -- among the 6.5 million prescriptions written globally since 2006.

Varenicline is part of a new class of smoking-cessation drugs known as neural nicotinic acetylcholine receptors. The medications affect cognitive functions like pain, mood, memory and attention by preventing nicotine from attaching to receptors in the brain.

Scientists believe these medications may have unique benefits like reducing withdrawal symptoms, and researchers at the National Institute on Drug Abuse are testing varenicline to determine its effectiveness in treating cocaine and alcohol dependence.

However, the side effects have raised serious concerns, with the Federal Aviation Administration last spring banning pilots and air traffic controllers from taking varenicline and the Food and Drug Administration following suit by adding strong warning language to varenicline's medication guide.

Pfizer says that it is difficult to trace the exact causes of the drugs because smokers as a group have higher than average rates of anxiety and depressive disorders, which suggests that mild or undiagnosed preexisting mental illness might have played a part in some of the reactions to the drug.

"It's a story that's still evolving, and it's very complicated," said Lorna Role, a researcher studying nicotinic receptors at Columbia University and Stony Brook. "So, going in with a drug like varenicline, I'm not surprised that there are side effects."