Which of the following is a common side effect of opioid analgesics?

Nausea and vomiting are indeed common side effects of opioid analgesics due to their action on the central nervous system and gastrointestinal tract. Opioids bind to specific receptors in the brain that can stimulate the vomiting center, leading to feelings of nausea and the act of vomiting. Additionally, opioids can cause delayed gastric emptying, which may exacerbate feelings of nausea.

The mechanism behind this side effect often involves interactions with the mu-opioid receptors, which, when activated, can influence the areas of the brain responsible for nausea. Patients using opioid medications may experience this side effect especially when they first start the medication or when the dosage is increased.

Other options like diarrhea are typically not associated with opioid use, as opioids often cause constipation instead. Increased appetite might not be a common side effect; in fact, many patients may experience decreased appetite. Insomnia can occur but is not a primary or common effect of opioids. Therefore, nausea and vomiting distinctly characterize the impact opioids can have on the gastrointestinal system, making this the correct choice.