WASHINGTON — The Food and Drug Administration has begun issuing alerts to patients and doctors on its Web site about emerging drug-safety concerns, including dangers posed by drugs prescribed for unapproved purposes.

The new site, called Drug Watch, marks a significant change in how the agency communicates risks to the public. The FDA in the past has spent months or years privately weighing and debating risk information with drug companies until a final determination was made on the scientific significance of the danger.

After being bashed by Congress and consumers for its slow response to emerging information about heart risks posed by blockbuster pain medications such as Vioxx, the FDA is beginning to make risk information available to the public much sooner.

"This is really a fundamental change," said Dr. Steven Galson, acting director of the FDA's Center for Drug Evaluation and Research. "We do intend to give companies a heads up before posting something new about their drugs, but we're not going to discuss it with them. They're not going to review it."

The safety alerts posted so far on the site — www.fda.gov/cder/drugSafety.htm — also are notable because they emphasize risks posed by doctors prescribing drugs for purposes never approved by the FDA. It's a practice called "off-label prescribing" because it involves using drugs in ways and to treat conditions not covered by the safety and effectiveness determinations on their FDA-approved labels.

"That's a very important aspect of this," Galson said. "As you know, we don't regulate the practice of medicine. We have no way of preventing physicians from using drugs off label."

But Galson said the agency does have a responsibility to let people know about known risks of off-label uses. "This is very controversial, of course," he said.

Some off-label drug uses are beneficial and based on good science, others have little proof of effectiveness and still others have been proved worthless in studies.

Officials at the American Medical Association, which represents doctors, and the Pharmaceutical Research and Manufacturers of America, which represents drugmakers, declined to comment, saying they are evaluating the Drug Watch site and will submit formal comments to the FDA.

Advocates for patients called the site an incremental move in the right direction, even though it requires patients to seek out the information on the Internet, rather than it being given to them when they pick up their medicines. The emphasis on early and off-label risk information is a departure for the FDA, they said.

"I don't think they had the political courage even a few years ago to take these kinds of steps," said Larry Sasich, a pharmacist with Public Citizen's Health Research Group, a consumer watchdog that publishes a searchable database of drug risks at www.worstpills.org.