Melenex was an anesthetic that was carried in 2260s Starfleet medkits.

In 2269, Doctor McCoy used melenex to make Spock appear ill, fooling the computer that controlled the Shore Leave Planet. McCoy described the effects of melenex as "brief unconsciousness and temporary skin discoloration," adding, "It looks worse than it is."

Once the doctor dosed Spock with ten ccs of melenex via hypospray, McCoy told Spock that it would take ten seconds to take effect, resulting in unconsciousness for less than five minutes. (TAS: "Once Upon a Planet")

In the novelization of "Once Upon a Planet" (as published in Star Trek Log 3), a drug called corpelomine replaces melenex and is said to take approximately twenty seconds to take effect, rather than the exact ten seconds.
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