γ-Butyrolactone (GBL) is a hygroscopic colorless, water-miscible liquid with a weak characteristic odor. It is the simplest 4-carbon lactone. It is mainly used as an intermediate in the production of other chemicals, e.g. methyl-2-pyrrolidone. In humans GBL acts as a prodrug for γ-hydroxybutyric acid (GHB), and it is used as a recreational CNS depressant with effects similar to barbiturates.