* German drugmaker seeking approval of "pink Viagra"

* FDA reviewers say drug failed to boost sex drive

* Agency staff also question risks

* Advisory panel to review drug on Friday

(Rewrites first paragraph to mention Viagra; adds byline and
comments from FDA, company, and women's group)

By Lisa Richwine

WASHINGTON (Reuters) - A pill designed to offer
women a counterpart to Viagra failed to boost female sex drive
in two studies, U.S. drug reviewers said in documents released
Wednesday.

Women who took Boehringer Ingelheim's experimental drug
flibanserin also reported depression, fainting, fatigue and
other problems. The drug's tolerability was "only moderate,"
Food and Drug Administration staff said.

An FDA advisory panel meets Friday to weigh whether to
recommend approval for flibanserin. The once-a-day pill,
nicknamed the "pink Viagra," is the latest attempt at a female
version of Pfizer Inc's successful blue pill for men.

Viagra and other male impotence drugs have failed to help
women.

Boehringer is seeking FDA approval to sell flibanserin by
prescription for treating premenopausal women with hypoactive
sexual desire disorder, defined as a lack of sex drive in
otherwise healthy women.

The proposed brand name is Girosa. It is not yet approved
anywhere in the world.

In a preliminary review, FDA staff outlined "major"
concerns on the company's effectiveness data. In two clinical
trails, the drug increased the number of sexually satisfying
experiences but failed to improve sexual desire.

"Therefore, neither study met the agreed-upon criteria for
success in establishing" efficacy, the reviewers wrote in a
summary prepared for the advisory panel.

Fatigue, drowsiness and sedation were "commonly reported"
and "it is not clear if labeling alone will be sufficient to
alert women to the numerous drug interactions that exist with
flibanserin," they said.

Depression, fainting and accidental injury also were more
frequent with the drug than a placebo.

Nearly 15 percent of women who took the recommended
flibanserin dose stopped taking it before the study ended due
to possible side effects, FDA staff added.

Flibanserin acts on neurotransmitters in the brain and was
originally tested as an antidepressant. FDA reviewers
recommended that the drug, if approved, carry a warning
required for all antidepressants about risks of suicidal
thoughts and behavior in children and young adults.

Boehringer, a privately held German drugmaker, said an
alternative analysis showed the drug did increase sexual desire
over one month. The company previously disclosed that women who
took the drug reported the number of satisfying sexual
experiences rose to an average of 4.5 per month from 2.8. For
placebo patients the rate jumped to 3.7.

"Women face extremely limited options when seeking help
for" low sex drive and need an approved treatment, the company
said in a summary prepared for the FDA panel.

Potential side effects were low and mild to moderate,
Boehringer said.

Pfizer's Viagra and other drugs to treat male impotence
work in men by widening blood vessels to increase the blood
flow needed for an erection.

Some critics question if flibanserin will appeal to women
if it wins FDA approval. They note women must take it every day
to maintain the effect on brain chemistry, while men take
Viagra only as needed ahead of sexual activity.

"Both of those things, that it's a chronic drug and that it
works on brain chemistry, raise important safety questions,"
said Amy Allina, program and policy director for the National
Women's Health Network.

Another U.S. drugmaker, BioSante, is developing a
testosterone skin gel to treat a decline in libido in
menopausal women. The company estimates the market for treating
female sexual dysfunction in the United States tops $2 billion
annually.
(Reporting by Lisa Richwine; additional reporting by Susan
Heavey; Editing by Maureen Bavdek and Tim Dobbyn)