What is a common side effect associated with opioid analgesics?

Opioid analgesics are commonly used for pain management, but one of the well-documented side effects associated with their use is constipation. This occurs due to the way opioids interact with the central nervous system and the digestive system. Opioids bind to specific receptors in the gastrointestinal tract, leading to decreased motility and slower transit time through the intestines. This results in harder stools and a greater likelihood of constipation.

The other options represent side effects that can occur with various medications or conditions but are not as uniquely associated with opioid use as constipation is. For instance, nausea may be a side effect, but it's more related to individual tolerance and can vary widely among patients. Increased appetite is generally not a side effect of opioids, whereas many patients may experience a decrease in appetite due to nausea or other factors. Insomnia can be influenced by pain levels and other health conditions rather than being a direct outcome of opioid use. Thus, constipation stands out as the most common and significant side effect specifically associated with opioid analgesics.