Why does inconsistent use of HIV medication lead to new, mutant strains?

Why does inconsistent use of HIV medication lead to new, mutant strains?T.L., Sudbury

The goal of taking drug ''cocktails," or combination therapy, with AZT, 3TC, protease inhibitors and other medications to combat HIV infection is to keep the AIDS virus from replicating, said Dr. Eric Rosenberg, an AIDS specialist and associate director of clinical microbiology at Massachusetts General Hospital.

When the levels of drugs in your system are high, the virus can't easily replicate. But if you miss doses, the levels drop, and the virus can mutate more readily, potentially becoming resistant to the drug.

''So long as the virus can't replicate, it can't mutate and resistance to a drug does not develop, or at least develops at a much, much slower rate," Rosenberg said.

Dr. Clyde Crumpacker, director of virology at Beth Israel Deaconess Medical Center, noted that it has taken years for doctors to appreciate the implications of all this: That the best way to keep the AIDS virus from replicating is to hit it hard with multiple drugs all at once, rather than waiting until a patient becomes resistant to one drug before starting another. ''This is something the developing world can learn from the American experience," he said. ''Americans had to learn it the hard way. Other countries can benefit from this."