Federal regulators under President Barack Obama have sharply shifted course on long-standing policy toward pharmaceutical residues in the nation’s drinking water, taking a critical first step toward regulating some of the contaminants while acknowledging they could threaten human health.

A burst of significant announcements in recent weeks reflects an expanded government effort to deal with pharmaceuticals as environmental pollutants:

_ For the first time, the Environmental Protection Agency has listed some pharmaceuticals as candidates for regulation in drinking water. The agency also has launched a survey to check for scores of drugs at water treatment plants across the nation.

_ The Food and Drug Administration has updated its list of waste drugs that should be flushed down the toilet, but the agency has also declared a goal of working toward the return of all unused medicines.

_ The National Toxicology Program is conducting research to clarify how human health may be harmed by drugs at low environmental levels.

The Associated Press reported last year that the drinking water of at least 51 million Americans contains minute concentrations of a multitude of drugs. Water utilities, replying to an AP questionnaire, acknowledged the presence of antibiotics, sedatives, sex hormones and dozens of other drugs in their supplies.

The news reports stirred congressional hearings and legislation, more water testing and more disclosure of test results. For example, an Illinois law goes into effect Jan. 1 banning health care institutions from flushing unused medicine into wastewater systems.

The EPA’s new study will look for 200 chemical and microbial contaminants at 50 plants that treat drinking water. The list includes 125 pharmaceuticals or related chemicals. This research will help federal water officials decide if regulations are needed.

In the first move toward possible drinking-water standards, the EPA has put 13 pharmaceuticals on what it calls the Contaminant Candidate List. They are mostly sex hormones, but include the antibiotic erythromycin and three chemicals used as drugs but better known for other uses.