Which side effect is most commonly associated with high doses of Fentanyl?

Bradycardia is the most commonly associated side effect with high doses of Fentanyl due to its action on the central nervous system. Fentanyl, an opioid analgesic, can induce a decrease in heart rate by activating opioid receptors, particularly the μ (mu) receptors, which can inhibit sympathetic output and promote parasympathetic activities. This results in vagal stimulation and consequently, can lead to a slower heart rate.

In comparison, while hypotension, increased heart rate, and vomiting can occur as side effects with various medications or conditions, they are not typically the hallmark response to Fentanyl at high doses. Hypotension may occur, but it's less directly related to Fentanyl than bradycardia, and the drug's primary effect on cardiac rhythm tends towards bradycardia rather than tachycardia or hypotensive responses. This makes bradycardia the more relevant side effect in this context.