Gamma-butyrolactone (GBL) is an industrial chemical that is illicitly used as a substitute and chemical precursor to gamma-hydroxybutyric acid (GHB) . GBL is converted to GHB in the body, which can turn off some functions of the nervous system. Despite serious safety concerns, people use GBL as a party drug, for athletic performance, depression, and other purposes, but there is no good scientific evidence to support these uses. GBL is classified as a Schedule I controlled substance, making it illegal in the US. GBL is closely related to GHB, and both are widely used as recreational drugs due to their mood-enhancing, stimulant, and sedative effects. GBL is often sold as a colourless, odourless, bitter or salty liquid, often in small bottles or vials, and can also come as a bright blue liquid known as ‘blue nitro’ or as a crystal powder. GBL is abused by teens and young adults as an alcohol substitute, at all-night parties and “raves,” and for enhanced sexual experiences. GBL is a prodrug of GHB, meaning it is converted into GHB in the body, and its effects are identical. However, it is two to three times as powerful as GHB, and its effects are felt faster, making it a far greater risk for overdose than GHB. It is very easy to overdose on GHB and GBL, and there is only a very small difference between the dose causing the desired effects and the dose leading to severe overdose.