WASHINGTON (AP) -- Federal health officials say Pfizer's anti-smoking drug Chantix did not increase psychiatric problems like depression and suicidal thoughts in two studies, though the findings are not definitive.

The Food and Drug Administration has been investigating reports of mood disorders and erratic behavior among Chantix patients since 2007. The agency said in a statement that two federally-funded studies involving more than 26,000 patients did not show an increased rate of psychiatric hospitalizations among Chantix patients, compared with those using nicotine patches and other treatments.

Regulators stressed that the studies only recorded psychiatric problems that resulted in hospitalization, which means many issues may have gone unreported. Additionally, the studies by the Department of Veterans Affairs and the Department of Defense were not large enough to pick up very rare side effects.