WASHINGTON (AP) - The government Friday approved the sale of a new soft contact lens considered safe enough for people to wear for a month without removal.

The lens allows six times more oxygen into the eye than current lenses — making it safer for the user to wear while sleeping.

Sleeping in contact lenses can increase risk for serious eye infections. Only a few extended-wear brands are approved for continuous wear, up to six nights - and some eye specialists worry about even that much overnight wear.

Also, eye doctors say many Americans continually wear contacts that aren't designed for extended wear.

Eye specialists said Ciba Vision's new Focus Night & Day lenses — designed to be worn continuously for up to a month and then discarded — promise to be an important advance. But they caution that they don't know just how safe the lenses really will be.

``They should be better, but you really have to take a guarded approach ... and watch patients with them,'' said optometrist Lee Raykovicz, contact lens chief at Johns Hopkins University's Wilmer Eye Institute.

Ciba Vision won Food and Drug Administration approval to sell the new lenses, and said they will be available by prescription next month. The company didn't announce an exact price, as retail markups vary widely, but estimated they will cost about $30 a month.

Several lens brands designed to be worn a month at a time sold in the late 1980s. Then a Hopkins study found that overnight use increased the risk of a serious eye infection called a corneal ulcer, which if left untreated could cause loss of vision. The risk was even greater for smokers who wore their lenses overnight.

Consequently, the FDA changed its rules and said extended-wear contact lenses were to be used only for a week at a time.

The risk with today's weekly contact lenses isn't large — about one in 500 users will get a corneal ulcer, said FDA optometrist James Saviola.

The problem: The lenses can block oxygen to the eye. That increases the swelling of the cornea — the clear tissue that covers the front of the eye — that normally occurs during sleep, and makes it more vulnerable to infection.

So when Ciba Vision created a new lens made of silicone that allows six times more oxygen to penetrate the eye than regular disposable contacts do, the FDA said it could sell if it proved no more risky than today's lenses.

Ciba Vision studied almost 1,400 people who wore either the new 30-day lenses or regular weekly lenses during one year. That study isn't large enough to capture the one-in-500 risk of a corneal ulcer, so the FDA checked for corneal inflammation, a milder infection considered a marker for the more serious risk.

About 4.7 percent of the 30-day wearers experienced that mild corneal infection, vs. 2.7 percent of the weekly users, a difference Saviola said wasn't statistically significant.

So the FDA approved sale of the lenses, but ordered Ciba Vision to conduct a 5,000-person follow-up study to make sure the more serious risk doesn't crop up.

The FDA has also created a question-and-answer brochure that all consumers are supposed to receive with the lenses to ensure they understand the potential risk.

Not everyone in Ciba Vision's studies could tolerate wearing the lenses for 30 days continually, but 67 percent wore them between 22 and 30 days, says the brochure, which will be posted on the FDA's Internet site next week.

Your doctor may recommend shorter wearing times ``and you should always adhere to his or her recommendations,'' the FDA brochure warns.

Once the lenses are removed, let the eyes rest for a full night before inserting a new pair, it warns.

Copyright 2001 The Associated Press. All rights reserved.