Which side effect is most likely to occur with high doses of morphine?

Urinary retention is a well-known side effect associated with high doses of morphine and other opioid medications. Morphine acts on opioid receptors in the central nervous system, which leads to various effects, including the modulation of pain and sedation. However, it also impacts the autonomic nervous system, particularly in its effects on the bladder and the ability to urinate. This is because morphine can inhibit the sensation of bladder fullness and reduce the tone of the bladder's detrusor muscle, leading to an inability to void effectively.

In contrast, while muscle rigidity, euphoria, and increased appetite can occur with morphine use, they are not as directly linked to high doses as urinary retention is. Muscle rigidity is more commonly associated with certain other opioids, especially at doses that lead to significant central nervous system effects. Euphoria can happen but is not a typical side effect that indicates a dangerous physiological response like urinary retention. Increased appetite is generally not recognized as a primary side effect of morphine and is less relevant compared to the risks associated with urinary retention in a medical context.