Even as Herceptin has become a hugely successful breast cancer drug, doctors have been troubled by a nagging question that has billion-dollar implications: how long should patients be treated with it?


Now the results of two closely watched studies announced Monday at a European cancer conference suggest an answer: neither a shorter nor a longer duration of treatment is better at staving off a recurrence of cancer than the current standard of one year.


The results mean that Roche, the drug’s manufacturer, has probably dodged a bullet, because a shift to shorter treatment could have meant the loss of $1 billion or more in sales. But Roche is also losing out on a windfall that would have resulted from a recommendation of longer treatment.


Herceptin, which has a wholesale price of about $54,000 a year in the United States, had global sales of 5.25 billion Swiss francs in 2011, or about $5.6 billion. Most use of the drug is aimed at reducing the chance that the cancer will come back after a patient with early stage breast cancer has surgery to remove the tumor.


One of the studies, sponsored by Roche itself, tested whether two years of treatment would be better than one. The study found no difference in the rate at which cancer recurred or in how long the patients lived. The results were to be presented Monday at the annual meeting of the European Society for Medical Oncology in Vienna.


The French National Cancer Institute studied whether six months of treatment would be as good as one year. If so, that would save insurers, government health programs and patients huge sums of money, and also possibly reduce the incidence of side effects from Herceptin, which can include heart problems.


The women who were treated for six months had a 28 percent greater risk of recurrence than those treated for a year. Strictly speaking, by the statistical measures used in the study, it could not be totally ruled out that six months of treatment might be roughly equivalent to a year. But the results do not seem likely to shift the standard of care to shorter treatment.


Still, efforts to find ways to use less Herceptin may not be over. It is possible that certain subgroups of patients would do just as well with shorter treatment. An analysis of such subsets will be presented in December, Xavier Pivot, an investigator in the trial and professor at the Université de Franche-Comté’, said in the news release from the European oncology society.


Moreover, there are a few other studies of shorter treatment under way in other European countries, including Britain and Italy.


The studies were started after a small study in Finland, published in 2006, found that a mere nine weeks of treatment with Herceptin reduced the risk of cancer coming back. And the benefit seemed roughly as large as that found in larger studies for one year of treatment.


Tim Anderson, an analyst at Sanford C. Bernstein & Company, estimated last month that in the worst case for Roche, a move to six months of treatment could reduce sales of Herceptin in 2016 by roughly $2.6 billion, lowering earnings per share that year by 12 percent. A move toward two-year therapy, by contrast, could have increased sales roughly $5 billion, lifting 2016 earnings per share by 25 percent.


Herceptin works by blocking the action of a protein called HER2/neu, or just HER2, that spurs tumor growth. The drug is used for only the roughly 20 percent of breast cancer cases in which the tumors make ample amounts of HER2.