Question 1: Which of the following substances is not classified as an opioid?
- A. Heroin
- B. Ketamine (Correct Answer)
- C. Methadone
- D. Fentanyl
Explanation: ***Ketamine*** - **Ketamine** is classified as a **dissociative anesthetic** and does not act on opioid receptors. - Its primary mechanism of action involves **N-methyl-D-aspartate (NMDA) receptor antagonism**. *Heroin* - **Heroin** (diacetylmorphine) is a **semisynthetic opioid** derived from morphine [1, 3]. - It rapidly crosses the **blood-brain barrier** and is metabolized to morphine, where it acts as a potent **mu-opioid receptor agonist** [3]. *Methadone* - **Methadone** is a **synthetic opioid** used in the treatment of opioid dependence and chronic pain [1, 2]. - It acts primarily as a **mu-opioid receptor agonist** with a long duration of action [1, 2]. *Fentanyl* - **Fentanyl** is a powerful **synthetic opioid** analgesic, much more potent than morphine [2]. - It selectively binds to and activates **mu-opioid receptors**, producing strong analgesic and sedative effects [2].