Which of the following is NOT a common alpha2-antagonist used to reverse alpha2-agonists?

Fentanyl is an opioid analgesic and does not function as an alpha2-antagonist. Alpha2-antagonists are specifically used to counteract the effects of alpha2-agonists, which are sedatives commonly utilized in veterinary anesthesia. Fentanyl works through a different mechanism by binding to mu-opioid receptors, providing pain relief rather than reversing sedation caused by alpha2-agonists.

In contrast, yohimbine, tolazoline, and atipamezole are all recognized as effective alpha2-antagonists. Yohimbine is derived from the bark of the yohimbe tree and is utilized for its ability to reverse sedation and bradycardia caused by alpha2-agonists. Tolazoline is used similarly, particularly in certain veterinary settings. Atipamezole is specifically developed for this purpose and is frequently employed to reverse the sedative effects of medetomidine and dexmedetomidine in animals. Understanding the specific roles of these drugs highlights why fentanyl is the correct answer as it does not belong to the category of medications used to reverse alpha2-agonist effects.