TRENTON, N.J. —  Two drug companies announced Monday they will collaborate on developing the first all-in-one, one-a-day pill to treat HIV (search) infection — a long-sought goal that would make it much easier for patients to stick with their medication.

Currently, the best AIDS treatment requires patients to take two to four pills a day. Less than a decade ago, many patients had to take 25 to 30 pills a day, often at precise times and under specific conditions such as with food, making it extremely difficult for patients to stick to the complex schedule. Missing doses makes it easier for the virus to mutate and become resistant to medication.

In the first collaboration by competing AIDS drug makers, Bristol-Myers Squibb Co. (search) and Gilead Sciences Inc. (search) formed a joint venture to test and market a single pill combining three widely used medicines.

Because the three individual drugs already are on the market, the once-a-day combination could be approved and on sale as early as the second half of 2006, said David Rosen, a spokesman for Bristol-Myers Squibb.

"To have it all in a single pill is terrific," Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases (search), told The Associated Press.

The combination pill will be made up of Sustiva, made by Bristol-Myers Squibb, and two AIDS drugs made by Gilead Sciences: Viread and Emtriva.

The latter two drugs are members of the same class of AIDS drugs, but they block copying of the AIDS virus at two different points early in its replication cycle. Sustiva is from a different class of drugs and attacks the virus later in the cycle.

The two companies will have to find a way to combine the component drugs so that the single pill is absorbed the same way, has identical effects, lasts in the body as long and has the same shelf life, said Robert Lipper, vice president of biopharmaceutics research and development at Bristol-Myers.

"It's the first time ever that two companies with competing products have worked together," said Dr. Michael Saag, director of the AIDS Center at the University of Alabama at Birmingham. "This is something patient advocates and a lot of physicians have been pushing for for over a decade."

Fauci said: "We hope it's the beginning of future collaborations."

The three drugs, which are already becoming the treatment of choice, together cost $900 to $1,000 a month. Rosen said it is too soon to say how much the single pill will cost.

The combination does not include a protease inhibitor, a class of drug that has been standard in recent years but that also carries many side effects.