Filed at 10:50 p.m. ET

NEW YORK (AP) -- At least 20 patients of the same doctor have been infected with hepatitis B, prompting city health officials to urge hundreds of others to get tested.

Those infected have recovered, and the source of their exposure is not clear, officials said. It's possible the outbreak is related to the ``improper administration of injectable medications from multi-dose vials,'' the city Health Department said in a news release.

More than 1,000 patients of Dr. Seymour Halpern have received letters advising them to seek testing for hepatitis B, hepatitis C and HIV, which can be transmitted in a similar way.

``While hepatitis B is not an uncommon infection, it is rarely transmitted in medical settings,'' said Dr. Marcelle Layton, the health department's assistant commissioner for communicable disease, in a statement.

Layton said there was no evidence of additional hepatitis B infections, but said symptoms may take up to six months to appear.

Halpern is cooperating with the investigation, which includes reviewing medical charts and interviewing patients, officials said.

The doctor's attorney, James Fabian, said in a statement there was no evidence patients were infected at the practice.

``Dr. Halpern does recognize, however, that in the interest of patient safety, it is appropriate to explore this possibility,'' the statement said.

Hepatitis B is transmitted through blood products, bodily fluids and needle pricks. It is a potentially fatal viral infection of the liver that can lead to cirrhosis and liver cancer.