The answer to this question would be: increasing fluid and fiber in the diet.
The opioid analgesic drug might be disrupting the nervous signal causing the reduction in the gastrointestinal mobility. If the intestine moves slower, then it means that the time it will absorb water will be increased. In this case, the feses will become harder because the water content will be lower. So it will be better if the fluid intake increased and take more fiber to make defecation easier.