A major maker of generic AIDS medicines voluntarily removed all its antiretroviral medications from the World Health Organization's list of approved drugs yesterday, a move that could potentially affect tens of thousands of AIDS patients in poor countries who have only recently entered treatment.

Ranbaxy Laboratories Ltd. of India removed seven drugs from the WHO list of "prequalified" medicines because it could not guarantee they were as potent as more expensive brand-name versions. The WHO reached a similar conclusion in August about three other drugs made by the company.

It is uncertain how much disruption the move will cause in clinics and hospitals in Africa, where a huge effort is underway to put people on life-sustaining "triple therapy" for AIDS. Global health organizations were scrambling yesterday to estimate the number of their patients who were taking Ranbaxy products and might need to switch to ones made by other companies.

A WHO official said that there is no evidence the Indian company's drugs are substandard but that the tests used to show that its generic AIDS drugs were as effective as the brand-name versions were unreliable or poorly documented.

Removal of the medicines from the WHO list does not make it illegal to use them. In fact, many AIDS programs in poor countries may conclude that interrupting treatment or switching medicines is more hazardous than continuing with the Ranbaxy pills.

The action, however, is another obstacle in the difficult effort to put 3 million people infected with human immunodeficiency virus (HIV) in poor countries on antiretroviral therapy by the end of 2005. The WHO's "3 by 5" target is already well behind in its timetable.

"It's likely to cause a lot of confusion in countries and ministries of health," said Paul Lalvani of the Global Fund to Fight AIDS, Tuberculosis and Malaria, which is providing money for AIDS programs in 95 countries, at least 10 using Ranbaxy drugs.

He added that the events may also cause some countries to question "the confidence that the [WHO] prequalification system was supposed to bring."

The Bush administration's $15 billion five-year global AIDS plan does not automatically recognize drugs on the WHO list. Instead, the administration has said that generic AIDS drugs bought with U.S. grants must be approved first by the Food and Drug Administration.

Ranbaxy said it hopes to finish the necessary tests to prove "bioequivalence" of its generics by January.

An official of the French medical charity Doctors Without Borders said the "de-listing" of Ranbaxy's drugs is unlikely to seriously affect its AIDS programs. The group has put 23,000 people on AIDS medicines in 27 countries, the most of any nongovernmental organization.

"We are not worried for the safety of our patients. We are confident in the drugs we are using. We can't make an equation that if a drug is removed from the list that it's not a good drug," said Daniel Berman of Doctors Without Borders' Paris office. He said the group can replace all but one of the Ranbaxy formulations with pills made by other companies, including the Indian generics firms Cipla and Hetero Drugs.

"The first priority we have is not to interrupt treatments. When possible, we will try to substitute the Ranbaxy products with others," he said. He called yesterday's action "a bit of a growing pain of prequalification," a system that he believes is highly useful to countries that do not have their own regulatory agencies for drugs.

UNICEF provides AIDS drugs to programs in 38 countries that treat about 10,000 patients, with more being added daily. Fewer than 10 of the countries are receiving Ranbaxy drugs, said Stephen Jarrett, deputy director of the supply division.

Eight pharmacists at UNICEF's drug operations in Copenhagen are determining the number of patients potentially affected and what the organization's advice will be.

At issue is whether Ranbaxy's products are absorbed by the body as well as the branded products are and whether they attain the same concentrations in the bloodstream.

WHO officials inspected the company in India that Ranbaxy hired to do bioequivalency tests and found its record-keeping lacking. The information "didn't deny or prove that there was no bioequivalence," said Daniela Bagozzi, a WHO spokeswoman.