Like all medications, this type of medication can cause side effects. The side effects will vary greatly from patient to patient in severity and length. Some patients may experience little to no side effects where other patients may experience several side effects. The side effects of this type of pain medication can include constipation (can get severe for some patients), drowsiness, trouble breathing, lightheadedness, (trouble urinating, fatigue, weakness, dry mouth and euphoria. Other side effects may also occur. In rare cases, an allergic reaction may occur. The signs of an allergic reaction can include difficulty breathing, hives, itching and rash.