RBP-6000 has proved more effective than a placebo in reducing opioid use in those suffering from a dependence on the product.
Shares jumped 19.4% higher to 359.6p on the news, despite the trials continuing until the first quarter of 2017.
Analysts at Numis are bullish forecasting sales of $500 million by 2020, if the treatment is launched in 2018 as expected.
They also believe that the treatment could become a blockbuster drug, taking around 30% of market share by volume.
The drug, which reduces cravings and addresses withdrawal symptoms, is a once a month injection, an improvement on existing product Suboxone, which involves a film dissolving in the mouth daily.
The film version of Suboxone had a 61% market share in the US at the end of June, so confidence that an extended release version will take market share is high.
A decision by US regulator the Food & Drug Administration (FDA) on whether or not the treatment can be used on the general public will be made within six months of filing.
RBP-6000 has been granted Fast Track status, due to it treating a condition where there is an unmet need. So it could be approved for sale by the end of 2017, if the remainder of the trials are as successful.
Indivior, a spin-out from consumer goods company Reckitt Benckiser (RB.), also has alcohol and cocaine addiction drugs under development.