A 50-year-old man with mild renal impairment and already taking morphine modified-release tablets is now being prescribed morphine 10 mg/5 mL solution for breakthrough pain. Which adverse reaction is most likely to be observed when taking or using the medication?

• A. Nausea
• B. Respiratory depression
• C. Euphoria
• D. Constipation

Answer: (C)

When a short-acting opioid is added on top of ongoing opioid therapy, the rapid rise in drug concentration can produce pronounced CNS effects before tolerances adjust. Euphoria is a subjective effect arising from opioid stimulation of reward pathways in the brain, and it is more likely to be noticed when the breakthrough dose causes a sharp peak in morphine levels from the solution. In someone already taking morphine in a regular regimen, tolerance tends to lessen many adverse effects like nausea or vomiting, and constipation tends to be a persistent, baseline issue rather than an acute breakthrough reaction. Respiratory depression remains a serious risk, especially with higher peaks or in renal impairment, but the most conspicuous acute adverse effect reported with a rapid breakthrough dose in an opioid-tolerant patient is euphoria.