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 agents. 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 aids 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."
Sopor Slime has also been found within pies in Gamzee's respiteblock. 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 among 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. It is more likely that the Sopor Slime's sedative effects kept Gamzee "stoned" or calmed, suppressing the more violent and sanguinary behavior typically associated with his blood color.
Hiveswap Friendsim has shown that Sopor Slime has minor healing properties, as Skylla tells the main character that it would help them with their burns.