What is common between heroin and fentanyl?

Both heroin and fentanyl share a classification as opioids, which is the key reason that this answer is correct. Opioids are substances that act on the opioid receptors in the brain to produce morphine-like effects, including pain relief and euphoria, but they can also lead to significant risks such as addiction and overdose.

Heroin is an illicit drug derived from morphine and is known for its potent effects and high potential for addiction. Fentanyl, on the other hand, is a synthetic opioid that is prescribed medically for severe pain management, but it is also known for its potency—many times stronger than heroin. The commonality of being categorized as opioids underlines both substances' impact on the central nervous system and their associated risks.

The incorrect options indicate different categories: stimulants are substances that typically increase alertness and energy levels; legal medications encompass drugs that are permitted for use under medical supervision and don’t specifically apply to both heroin and fentanyl; and hallucinogens primarily alter perception, mood, and cognitive processes, which is not characteristic of either heroin or fentanyl.