Which statement best describes morphine’s lipophilicity and intrathecal duration?

Which statement best describes morphine’s lipophilicity and intrathecal duration?

• A. Morphine is the least lipophilic opioid and has the longest duration of action intrathecally due to low lipophilicity.
• B. Morphine is the most lipophilic opioid and has the shortest intrathecal duration.
• C. Morphine’s intrathecal duration is unrelated to lipophilicity.
• D. Morphine does not cross the blood-brain barrier.

Answer: (A)

Understanding this question comes down to how lipophilicity influences intrathecal opioid duration. Hydrophilic opioids stay in the cerebrospinal fluid longer and diffuse into the spinal cord more slowly, so they provide longer-lasting analgesia when given intrathecally. In contrast, highly lipophilic opioids are absorbed quickly into the blood from the intrathecal space, leading to a shorter duration of spinal analgesia even though they may act rapidly. Morphine is relatively less lipophilic compared with other common intrathecal opioids like fentanyl or sufentanil, so it stays in the CSF longer and thus has a longer intrathecal duration. This aligns with the statement that morphine is the least lipophilic among the typical options and has the longest intrathecal duration. The other ideas—that duration is unrelated to lipophilicity, that morphine is the most lipophilic, or that it fails to cross the blood–brain barrier—do not fit the pharmacokinetic reality.

Understanding this question comes down to how lipophilicity influences intrathecal opioid duration. Hydrophilic opioids stay in the cerebrospinal fluid longer and diffuse into the spinal cord more slowly, so they provide longer-lasting analgesia when given intrathecally. In contrast, highly lipophilic opioids are absorbed quickly into the blood from the intrathecal space, leading to a shorter duration of spinal analgesia even though they may act rapidly.

Morphine is relatively less lipophilic compared with other common intrathecal opioids like fentanyl or sufentanil, so it stays in the CSF longer and thus has a longer intrathecal duration. This aligns with the statement that morphine is the least lipophilic among the typical options and has the longest intrathecal duration. The other ideas—that duration is unrelated to lipophilicity, that morphine is the most lipophilic, or that it fails to cross the blood–brain barrier—do not fit the pharmacokinetic reality.