Final answer:
Methadone is the opioid with a very long context-sensitive half-life, making it less suitable for use as an infusion in situations requiring rapid adjustment of analgesic effects.
Step-by-step explanation:
The opioid with a very long context-sensitive half-life that limits its use as an infusion is Methadone. This characteristic makes it less ideal for scenarios where rapid changes in the analgesic effect are needed, as its extended half-life can lead to prolonged effects and potential accumulation in the body. Methadone is an example of a synthetic opioid which is often used for pain management and in treatment programs for opioid dependence. However, for acute pain management, opioids with a shorter context-sensitive half-life are often preferred due to their easier titration and quicker offset of action when the infusion is discontinued.