English: Benzylfentanyl (R-4129) is a fentanyl analog. It was temporarily placed in the US Schedule I by emergency scheduling in 1985 due to concerns about its potential for abuse as a designer drug, but this placement was allowed to expire and benzylfentanyl was formally removed from controlled substance listing in 2010, after the DEA's testing determined it to be "essentially inactive" as an opioid. Benzylfentanyl has a Ki of 213 nM at the mu opioid receptor, binding around 1/200 as strong as fentanyl itself, though it is still slightly more potent than codeine.