Which of the following is a characteristic of opioids?

Opioids are indeed slightly modified versions of opium, which is where they originate. Opium is derived from the poppy plant and has been used for centuries for its pain-relieving properties. Opioids, such as morphine and codeine, are specially synthesized to strengthen certain effects of opium or to create new drugs that resonate with the body’s natural receptors for pain relief and euphoria. This modification may enhance their efficacy as pain management medications while potentially increasing the risk of addiction.

The other options do not align with the fundamental characteristics of opioids. For instance, opioids do not stimulate brain activity; rather, they primarily depress the central nervous system, leading to sedation and relaxation. Additionally, opioids do not enhance alertness; their effects are often sedating. Lastly, opioids are generally known to carry a risk of addiction, which contradicts the notion of them being primarily non-addictive. Understanding these nuances helps clarify why the selected answer stands out in relation to opioids' properties and usage.