Which symptom is characterized as a side effect of opioid use?

The symptom characterized as a side effect of opioid use is hallucinations. Opioids, while effective for pain relief, can influence the central nervous system and lead to alterations in perception, mood, and consciousness. Hallucinations may manifest as seeing or hearing things that are not present, which is a more severe side effect associated with higher doses or certain individuals’ reactions to opioids.

The other options provided do not typically align with the common side effects of opioid use. Weight gain is not a known consequence; in fact, opioids can affect appetite and may lead to weight loss in some individuals. High blood pressure is not a common side effect associated with opioids, as they are more likely to cause sedation and potential hypotension due to their relaxing effects. Finally, increased activity level contradicts the sedative effects of opioids, which usually lead to decreased activity and a tendency towards lethargy. Thus, hallucinations stand out as a notable side effect linked to the use of opioids.