Alfentanil (R-39209, trade name Alfenta, Rapifen in Australia) is a potent but short-acting synthetic opioidanalgesicdrug, used for anaesthesia in surgery. It is an analogue of fentanyl with around 1/4 to 1/10 the potency of fentanyl and around 1/3 of the duration of action, but with an onset of effects 4x faster than fentanyl. Alfentanil has a pKa of approximately 6.5, which leads to a very high proportion of the drug being uncharged at physiologic pH. This unique characteristic is responsible for its rapid onset. It is an agonist at mu opioid receptors.
While alfentanil tends to cause fewer cardiovascular complications than other similar drugs such as fentanyl and remifentanil, it tends to give stronger respiratory depression and so requires careful monitoring of breathing and vital signs. Almost exclusively used by anesthesiologists during portions of a case where quick, fast acting (though not long lasting) pain control is needed (i.e. during a nerve block, head pinning etc..) Alfentanil is administered by the parenteral (injected) route for fast onset of effects and precise control of dosage.
Alfentanil is a restricted drug which is classified as Schedule II in the USA, according to the U.S. DEA website.