Broadly they are referred to as substances of abuse, either in a pure form or a preparation, that are not controlled under the 1961 Convention on Narcotic Drugs or the 1971 Convention on Psychotropic Substances, but which may pose a public health threat. Some NPS may be under a patchwork of national controls.

These substances are often deliberately chemically engineered to circumvent existing international and domestic drug control measures. NPS can be man-made, synthetic substances or natural materials and generally encompass several groups of substances, such as synthetic cannabinoids, phenethylamines, synthetic cathinones, tryptamines, fentanyl related substances etc.