WASHINGTON — A long-awaited study has found that two of the main medications for treating opioid addiction are similarly effective, a finding likely to intensify the hard-fought competition between drugmakers seeking to dominate the rapidly expanding opioid treatment market.

The study, funded by the federal government, compared Vivitrol, which comes in a monthly shot and blocks the effects of opioids, and Suboxone, which is taken daily in strips that dissolve on the tongue and contains a relatively mild opioid that helps minimize withdrawal symptoms and cravings.

Researchers found that 52 percent of those who started on Vivitrol relapsed during the 24-week study, compared with 56 percent of those who started on Suboxone.

But the study, conducted with 570 adults addicted mostly to heroin, also found a substantial hurdle for Vivitrol. Because the medication can be started only after a person is completely detoxed from opioids — a process that can take over a week — more than a quarter of the study participants assigned to Vivitrol dropped out before being able to take their first dose. Suboxone can be started shortly after withdrawal symptoms begin, and only six percent of those assigned to take that drug dropped out before taking an initial dose.