UPDATE 3-U.S. orders warning on Pfizer, Glaxo smoking drugs

* FDA: Chantix, Zyban need to warn of mental problems

* Agency orders further clinical trials

* Pfizer says Chantix benefits outweigh risks

* Chantix alert on accidents made more prominent (Adds further reaction, edits)

By Susan Heavey

WASHINGTON, July 1 (Reuters) - U.S. health officials on Wednesday ordered Pfizer Inc (PFE.N) and GlaxoSmithKline PLC (GSK.L) (GSK.N) to add strong "black box" warnings on their anti-smoking drugs to highlight the risk of serious mental health problems, including suicidal thoughts.

The warnings, which must be added to Pfizer's Chantix and Glaxo's Zyban, follow more than five thousand reports of depression, hostility and other behavioral changes, the Food and Drug Administration said.

FDA officials said there was still a need for the medications, their concerns tempered by smoking's health dangers, such as lung cancer and heart disease.

"We don't want to scare people off from trying to use the medication to stop smoking. We just want them carefully monitored," said Dr. Curt Rosebraugh, who oversees the FDA office that reviews smoking-cessation drugs.

Additionally, the FDA strengthened its caution about possible accidents and serious injuries with Chantix linked to loss of consciousness, mental confusion, dizziness and muscle spasms. But it stopped short of adding it to the boxed warning as some health advocates had urged.

Rosebraugh told reporters the FDA could not directly link the reported problems to the drugs, adding that withdrawal from nicotine in tobacco can also cause problems.

The agency is also requiring the companies to conduct additional clinical trials to help determine how serious a problem the mood disorders are, including among patients who already have depression or other similar conditions.

The warnings will limit Pfizer's ability to run some types of advertisements for Chantix, which the world's largest drugmaker had been counting on to help revive profits.

Instead, Pfizer has seen the drug's sales plunge over the last year amid safety concerns. Global first-quarter sales of Chantix fell 36 percent to $177 million.

THOUSAND OF REPORTS

FDA first alerted patients and doctors about possible mood changes with Chantix in November 2007, and has continued to receive reports of problems.