Naltrexone is an opioid receptor antagonist that, like naloxone, has a marked affinity for µ opioid receptors. Since the late 1970s (2), several studies have suggested that panic attacks might be the consequence of hyperactivity of locus ceruleus noradrenergic cells. These neurons possess µ opioid receptors and are innervated by inhibitory fibers containing both enkefalins and endorphins (3). Similarly, exogenous opioid agonists, acting at µ receptors, produce inhibition of locus ceruleus cells (4). This pharmacological inhibition is reversed by opioid antagonists such as naloxone, which in turn produce hyperactivity of noradrenergic cells (5). In subjects in whom overactivity of endogenous opioids occurs, naltrexone might induce panic attacks by removing this inhibitory effect.