Johnson & Johnson said Thursday that its new drug for prostate cancer was effective when used earlier during treatment than it is now, potentially broadening its use and also posing a competitive threat to a rival drug, Dendreon’s Provenge.

Johnson & Johnson’s drug, called Zytiga, was approved in April for use in men with advanced prostate cancer who had already tried chemotherapy using the drug docetaxel.

But the new trial showed that the drug was also effective when used before chemotherapy. Men who received Zytiga went significantly longer before their disease progressed than those who received a placebo. They also lived longer on average, by what the company called a “clinically meaningful” amount, although the company’s description suggested that this difference fell short of being statistically significant.

An independent committee monitoring the trial concluded that it would be unethical to continue; it effectively ended the trial early so that the patients getting the placebo could be offered Zytiga instead.

Many patients and their doctors would prefer to use Zytiga, a pill, before they try chemotherapy. But some insurers have been reluctant to pay for the unapproved use, since Zytiga costs $5,000 a month.

The results of the new trial might change that. They could also lead to approval of Zytiga for use before chemotherapy, which would mean potentially more patients and a longer duration of treatment for Johnson & Johnson.

“An approval in the prechemo population should more than double the market opportunity for Zytiga,” Derrick Sung, an analyst at Sanford C. Bernstein & Company, said in a note.

But it could narrow the market opportunity for Dendreon’s Provenge, which is now used before chemotherapy.

Shares of Dendreon, a Seattle biotechnology company, fell 75 cents, or 7 percent, to close at $10.12. Johnson & Johnson shares rose 55 cents, or almost 1 percent, to $64.85.

Clinical trials have shown that Provenge, which trains the body’s immune system to attack the cancer, extends median survival about four months. But the drug is expensive, at $93,000 for a course of treatment, and cumbersome to administer, making a pill like Zytiga an attractive alternative, doctors and analysts have said.

Still, Johnson & Johnson on Thursday did not release any data on the results of the trial, saying that would be presented at a medical meeting.

Some analysts said Zytiga’s lack of a statistically significant survival benefit could help Dendreon. It is still possible that the survival benefit will become statistically significant as the patients in the trial are followed over time, though that is somewhat less likely because many of the placebo patients will now start on the drug.

Analysts said the success of Zytiga could also mean a somewhat similar drug being developed by Medivation and Astellas Pharma might prove successful for prechemotherapy patients in its clinical trial. The drug, MDV3100, has been shown to prolong survival for postchemotherapy patients but has not yet received regulatory approval.

Shares of Medivation rose $8.72, or 13.6 percent, to $72.91.

Johnson & Johnson’s trial involved 1,088 men whose cancer had spread beyond the prostate and was no longer controlled by hormone-deprivation therapy. The men had mild symptoms or no symptoms.

Patients in both arms of the trial took the steroid prednisone, which is given with Zytiga to reduce potential side effects.