The U.N. agency issued new guidance advising doctors to start giving patients AIDS drugs a year or two earlier than usual. The advice could double the number of people worldwide who qualify for treatment, adding three million to five million more patients to the five million already awaiting AIDS drugs.

WHO's previous HIV treatment advice was published in 2006. Since then, several studies have shown that people with HIV who start drugs earlier than recommended have a better chance of surviving.

WHO now advises doctors to start HIV patients on drugs when their level of CD4 cells - a measure of the immune system - is about 350. Previously, they said doctors should wait until patients' levels hovered about 200. In most Western countries, doctors start treating HIV patients when their CD4 count is about 500.

David Ross, an AIDS expert at the London School of Hygiene and Tropical Medicine, said there was compelling evidence that HIV patients should start treatment sooner. People with HIV who are not on AIDS drugs, he said, are more likely to catch a potentially fatal disease such as tuberculosis or develop other complications when they do start the drugs.

WHO's new recommendations also advise pregnant women with HIV to take the drugs earlier and while breast-feeding. It also said countries should phase out the use of the commonly used AIDS drug stavudine because of its toxic side effects.

If countries with large outbreaks adopt the guidance, many more people could live longer, healthier lives, Hiroki Nakatani, a top WHO official, said in a statement.

Still, WHO's advice raises questions about how countries and donor agencies will pay for the lifelong AIDS treatment. It may also be difficult to persuade HIV patients to start the drugs sooner, when some may not have any AIDS symptoms. Putting more patients on the treatment for a longer period could also encourage drug resistance.

Some experts said the new guidelines could add billions to the cost of global AIDS programs.