Alfentanil (R-39209 trade name Alfenta Rapifen in Australia) is a potent but short-acting synthetic opioid analgesic drug 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.
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