A 78-year-old woman with COPD, heart failure, and chronic pain is on Morphine Sulphate Modified Release 30 mg BD, Paracetamol 1 g QDS, Furosemide 40 mg, and Salbutamol inhaler PRN. She reports severe constipation despite Senna 15 mg at night. She has not opened her bowels for 5 days. According to prescribing guidelines, what is the MOST important principle regarding laxatives in patients on regular opioids?