In 1994 zopiclone (Imovane), a cyclopyrrolon, was introduced in Norway as a new kind of hypnotic. In 1996 zopiclone had a 26% share of the hypnotic market. This review of relevant literature has revealed a lack of documentation on the adverse effects of zopiclone. The similarities between zopiclone and benzodiazepine hypnotics are more striking than the differences. The bulk of comparative research has been carried out with triazolam, a drug taken off the Norwegian market in 1991. With zopiclone there is less inhibition of psychomotor function the day after intake than with flunitrazepam. Zopiclone causes less subjective "hangover" than nitrazepam, but there is a similar inhibition of psychomotor function the day after intake, and in some cases greater addictive potential.