Final answer:
Constipation is the opioid side effect that does not lessen over time, as opposed to other side effects like nausea and sedation, to which the body can develop tolerance.
Step-by-step explanation:
The opioid side effect that does not lessen over time is constipation. Opioids can lead to a number of side effects like nausea, sedation, and analgesia. Of these, patients typically develop a tolerance to nausea and sedation, meaning that these side effects can become less pronounced as the body adjusts to the medication. However, tolerance does not generally develop for the side effect of constipation, making it a persistent issue for individuals using opioids over a long period of time.