Anxiolytics and sedatives are used in current anaesthetic practice for anxiolysis before surgery and as adjuvants during anaesthesia. The safety profile of these agents depends on their pharmacokinetic and pharmacodynamic profiles, patient comorbidity and the experience of the clinician. Sedative drugs have the potential to cause severe respiratory depression, and hence they should be used only with standard physiological cardiorespiratory monitoring. The potential for respiratory depression is especially high for procedural sedation administered by non-anaesthetists in remote locations. Drugs used for anaesthesia vary in their pharmacology, but have broadly similar clinical effects. The choice of drug is usually a matter of individual preference, although pharmacokinetic and pharmacodynamic parameters influence the choice of anaesthetic agents, especially in day surgery. Most intravenous agents are thought to alter consciousness by an effect at the γ-aminobutyric acid (GABAA) or N-methyl-d-aspartic acid (NMDA) receptors or both. Our understanding of the mechanisms of action of anaesthetic drugs is incomplete, not least because of a lack of understanding of consciousness. Several theories have been proposed during the past century, but none has elucidated the processes involved. With the use of modern imaging techniques, anaesthetic-drug action may be better understood, leading to a better understanding of consciousness and cognitive functions.