(Reuters) - Drugmaker Indivior on Thursday reported a fourth-quarter loss and forecast lower 2020 revenue after its top opioid addiction treatment lost more than half its market share to cheaper rivals in its biggest market, the United States.Net loss was $55 million (£42.4 million)for the three months ended Dec. 31, compared to a profit of $24 million a year earlier, the company said, as it also took a hit from higher marketing costs to promote its newer injectable treatment.