Filed at 3:12 a.m. ET

GAITHERSBURG, Md. (AP) -- An experimental pill taken as a cold begins can clear up runny noses a day sooner than usual -- but Picovir faces a big hurdle before it goes on pharmacy shelves.

Government scientists decided it may not be safe enough to be taken by millions of Americans.

Even a one-day reduction in sniffles may be important for many people, advisers to the Food and Drug Administration acknowledged.

But because the common cold typically is a weeklong annoyance and not a real danger, any treatment must mount ``a very substantial bar'' of safety proof, panelist Dr. L. Barth Reller of Duke University said Tuesday.

So far, the advisers decided unanimously, Picovir hasn't met that test. One of the biggest problems is that Picovir appears to reduce the effectiveness of birth control pills so much that some women might accidentally become pregnant.

They also worried that attacking just colds could cause dangerous mutations among related viruses in the family Picovir targets, bugs that cause more serious infections like meningitis.

``By treating a relatively mild disease ..., if we have serious infections down the road, we might not be able to treat'' them, FDA adviser Dr. Jonathan Schapiro of Stanford University said.

Today's over-the-counter remedies ease symptoms by temporarily drying up runny noses or suppressing cough. Picovir maker ViroPharma Inc. hopes its drug, known chemically as pleconaril, will become the first prescription remedy to attack the actual virus that causes many, although not all, colds.

The FDA isn't bound by its advisers' recommendations but usually follows them. A final decision is expected by summer.

Picovir began generating excitement last winter when scientists reported a study of 2,000 adult cold sufferers. Overall, those given Picovir recovered a day faster -- in an average of six days -- than people who took dummy pills. Some patients started to feel better within a day of starting treatment.

FDA's advisers pointed out a list of caveats:

--Picovir worked only if taken within the first 24 hours of the start of sniffles. It had to be taken three times a day, with meals.

--Smokers, who can have longer, more serious colds than other people, got no benefit.

--No one knows yet if it works or is safe for children, who get colds far more often than adults; those studies are going on now.

--Nor is there information on minorities, the elderly or people with other ailments, such as asthma or heart disease, because Picovir was studied only in otherwise healthy, young adults with colds.

ViroPharma also is studying Picovir as a way to prevent colds, by taking the pills daily for six weeks. That study revealed the FDA advisers' biggest misgivings: Picovir can reduce by a third the amount of estrogen from birth control pills that circulates in a woman's body. That led a significant number of birth control users to bleed between their menstrual periods, and two of them became pregnant, despite advice to use backup contraception while taking Picovir.

Birth control pills can fail on their own, noted Dr. Daniel Mishell, a contraceptive expert from the University of Southern California, a consultant for ViroPharma. Also, Picovir caused the ``breakthrough bleeding'' by speeding estrogen's metabolism, but Picovir didn't affect the equally important hormone progestin also found in birth control pills, he said. Thus, ``I would expect it to not have any effect on pregnancies.''

``We don't have hard data to know what the risk is,'' said FDA analyst Russ Fleischer. He said if Picovir were to be approved, women would need clear warnings to use backup contraception.

Also, some people who took the asthma drug theophylline together with Picovir experienced heart palpitations, and it's unclear how many more drugs Picovir could interact with, some advisers said.

Picovir attacks the picornavirus family, which includes the rhinovirus that causes about half of all colds. Rhinovirus is the dominant cause of colds in the spring, summer and early fall, while other viruses predominate in late fall and winter.

Half of cold sufferers don't have picornaviruses, and there's no way for doctors to tell in advance who has which virus and thus should get Picovir, said FDA adviser Dr. Fred Gordin of George Washington University. Nor, he said, did Picovir reduce the rare but problematic complications of colds such as earache or bronchitis.

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On the Net:

ViroPharma Inc.: http://www.viropharma.com

Food and Drug Administration: http://www.fda.gov