Sopor Slime is a substance mainly used in troll Recuperacoons. Its specific properties are not described in great detail; but it appears to be a variety of sedating agent. Its name is derived from the term "soporific", meaning "to cause drowsiness or induce sleep", which is also used as a term for a drug that has the same effect. It is stated that submergement into Sopor Slime aides sleep and helps "assuage the terrible visions of blood and carnage that plague the dark subconscious of [their] species". These terrible visions may be a result of the subjugglator's psychic powers, or "chucklevoodoos." In a formspring answer, Hussie said that Gamzee had made the doll in John's dream room on Prospit by amplifying his fears, and that the other purplebloods may have similar powers which would explain their ability to have such control over the population.
Sopor Slime has also been found within pies in Gamzee's respite block. It is noted that it does "weird things to a troll's head", and that it is not made for consumption, explaining the typically "stoned" behavior of Gamzee. This suggests that it possesses psychoactive properties in addition to its ability to sedate. It's never elaborated if baking Sopor Slime pies is a practice unique to Gamzee, or if it is a more common practice amongst trolls, comparable to human use of marijuana.
It is unclear if ingesting Sopor Slime has any long-term effects on a troll's physical or mental health, but Gamzee mentions that it "RUSTS YOUR MOTHERFUCKING THINK PAN" after he kicks his drug habit. It's however unlikely that Gamzee's prior consumption of Sopor Slime caused his rampage of insanity. In fact it is more likely that the Sopor Slime's sedative effects kept Gamzee "stoned" or calmed, suppressing the more violent and sanguinary behaviour typically associated with his blood color.
A possible origin of Sopor Slime may be the Alternia's limebloods. Calliope mentions that limebloods were hunted to extinction; Sopor Slime is a green substance widely consistent across the population of Alternia. However, it should be noted that lime blood had a vastly different colour. This connection could also be a reference to Soylent Green.