WASHINGTON - It's about to get tougher to buy the acne drug Accutane, as the government imposes a last-ditch effort to prevent birth defects caused by the skin-clearing pills.

Starting at year's end, thousands of Accutane users - plus doctors who prescribe it and stores that sell it - must enroll in a national registry that ensures they understand all the drug's risks, and take special steps to avoid the most notorious one, Accutane-damaged pregnancies.

Critics see it as the drug's last chance, after two decades of safety warnings and other restrictions failed.

"We are very happy that this has happened and wish it had been done years ago," said Dr. Nancy Green, medical director of the March of Dimes, which long pushed for the change.

But the group will closely monitor whether the unprecedented sales curbs are enough to protect babies. If not, "we may very well call for it to be taken off the market," Green warned.

The Food and Drug Administration announced the new iPLEDGE program Friday, more than a year after its own scientific advisers urged the move.

"This is a system that has been long in the works and many would say is long overdue," acknowledged Dr. Sandra Kweder, FDA's deputy drug chief. "The responsibility for ensuring that fetuses are not exposed to this medicine is enormous, and is shared by everyone" who takes, prescribes or sells it.

If a woman becomes pregnant while taking the acne drug, her baby can suffer severe brain and heart defects, mental retardation and other abnormalities, even if the mother took only a small dose for a short period. That's a risk for 30 days after stopping the drug, too.

But that's not the only side effect. The drug also may trigger or worsen depression or suicidal thoughts, a warning on the pills' label that FDA strengthened Friday and that patients are to receive as part of the new program.

Under iPLEDGE, every patient - man or woman - must enroll in a computerized registry starting Dec. 31 to receive Accutane or generic versions of the drug isotretinoin.

Doctors must register, too, if they wish to continue prescribing the pills. Drugstores and wholesalers also must register in order for the pills' manufacturers to ship them any supplies.

While the FDA has imposed patient registries in an effort to control side effects from other troublesome drugs, this is the largest and most complex ever tried.

All patients seeking a prescription must sign a document informing them of all of Accutane's side effects, agree to monthly doctor visits for refills and agree not to share the pills with anyone. Then the doctor registers each patient into the iPLEDGE database, giving him or her a special identifying code number, and writes the prescription.

For women of childbearing age, there are additional steps.

- They must undergo two tests to ensure they're not pregnant in a laboratory or doctor's office before the initial prescription, and monthly pregnancy testing before each refill. Home pregnancy tests don't count. The doctor must enter the pregnancy test results into the iPLEDGE system, and the woman must buy her pills within seven days.

- These women also must agree to use two forms of birth control while using Accutane, and must self-register on iPLEDGE - in addition to the doctor's initial registration - to report that they're doing so. There's no way to know if women lie in this step, but FDA sees it as an additional warning opportunity.

The pharmacist must check the database before filling a prescription to ensure that patients followed the rules. Accutane maker Hoffman-La Roche and generic manufacturers will monitor drugstores and wholesalers, and are supposed to cut off violators' sales supplies.

The curbs are overdue, and don't do enough to ensure that patients understand the psychiatric risks, said Rep. Bart Stupak, D-Mich., whose teenage son committed suicide while using Accutane.

Nor will they curb overprescribing, he said. Accutane is supposed to be used for severe acne only, but is widely acknowledged to be prescribed for more minor cases. The FDA estimates that 100,000 prescriptions are filled each month.

Since Accutane began selling in 1982, the FDA has reports of well over 2,000 pregnancies among users. The vast majority ended in abortion or miscarriage, but the FDA counts more than 160 babies born with drug-caused defects. Critics note there likely were many more pregnancies because doctors haven't been required to report Accutane-linked pregnancies. Friday's new rules mandate that they do.

The registry actually opens on Aug. 22, giving users time to learn how it works before the provisions become mandatory at year's end. On that date, patients may enroll at http://www.ipledgeprogram.com or by phone at 1-866-495-0654.

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On the Net:

FDA Accutane info: http://www.fda.gov/cder/drug/infopage/accutane/default.htm