An injectable liquid composition for the treatment of a physical addiction includes naltrexone in the amount of 5%-95% of the total weight of the composition at least two co-polymers in the amount of between 5% -95% of the total weight of the composition and a biocompatible water miscible organic solvent in the amount of 5% to 50% of the total weight of the composition. The composition is adapted to form a gel-like mass when contacted with an aqueous environment and to dissolve at a predetermined rate to release a therapeutic naltrexone dose over a predetermined period of time.
