A soporific was a chemical that induced sleep. To access a "conference call", Madame Vastra and Jenny Flint used soporific-laced candles. They also sent a soporific candle to Clara Oswald, along with a letter laced with soporific in case she didn't use the candle. (TV: The Name of the Doctor)
A soporific agent was used by the Soul Pirates when kidnapping children. Susan was exposed to the agent when trying to save some children she had become acquainted with. (PROSE: A Big Hand for the Doctor)