The World Health Organization’s dosage guidelines for two leading tuberculosis medications may be far too low for patients with H.I.V., allowing them to remain contagious for longer than necessary, a new study has found.

TB, now the leading infectious killer worldwide, takes over 1.5 million lives per year. Treatment lasts at least six months and can cause serious side effects, making it difficult for patients to stick to it.

Doctors have been prescribing two TB drugs, rifampicin and isoniazid, for almost half of a century. But the new research, published in March in the journal Clinical Infectious Diseases, adds to growing evidence that higher doses may kill the deadly mycobacteria faster, curbing transmission.

“I’m not surprised,” said Dr. Melvin K. Spigelman, the president of the Global Alliance for TB Drug Development. “The recommended doses were really based on the cost of the drug, not on good science that showed it was the right dose. People, understandably, tried to get away with the least amount that seemed like it worked.”