European regulators have lifted a warning on Pfizer's smoking cessation drug Champix - called Chantix in the United States - after a clinical study concluded it did not appear to increase the risk of neuropsychiatric side effects.

Champix is approved as an effective therapy for helping smokers quit but the European Medicines Agency added a "black triangle" warning following reports linking it to agitation, aggression, panic, anxiety, depression and suicidal thoughts.

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The special warning required additional safety monitoring for patients taking the medicine.

Pfizer said on Monday that experts at the European drugs watchdog had now adopted a decision to change the label in the light of results from a large study, published in The Lancet medical journal in April, that provided safety reassurance.

Pfizer's medicine, which has been prescribed to millions of patients around the world, also carries a "black box" warning in the United States relating to neuropsychiatric adverse events.

A Pfizer spokesman said the company was awaiting feedback from the U.S. Food and Drug Administration on the matter.