Sedative: A drug that calms a patient, easing agitation and permitting sleep. Sedatives generally work by modulating signals within the central nervous system. If sedatives are misused or accidentally combined, as in the case of combining prescription sedatives with alcohol, they can dangerously depress important signals that are needed to maintain heart and lung function. Most sedatives also have addictive potential. For these reasons, sedatives should be used under supervision and only as necessary.