Injectible microcapsules containing narcotic antagonists have been prepared with dl-poly (lactic acid) as the coating material. The encapsulation technology has been developed to the point that high yields of less than 180 mu capsules can be prepared routinely. Such capsules with an initial payload of 50 wt. per cent naltrexone pamoate provide 60-90 per cent antagonism to the action of morphine 28 days after injection into mice as a peanut oil/aluminum monostearate suspension at a dose level of 40 miligrams naltrexone pamoate/kg. mouse.