Behavioural toxicity is relatively common among medicinal drug users and evidence shows that drugs frequently produce adverse effects that prevent their users from performing everyday operations in a normal manner. Epidemiological research generally indicates that the use of sedative drugs is associated with an increased risk of becoming involved in injurious accidents. Empirical studies have also demonstrated adverse effects of sedative drugs on the performance of healthy volunteers and patients in laboratory tests designed to measure psychomotor and cognitive function, and in real life-tests measuring on-the-road driving performance. Empirical studies also indicate that behavioural toxicity can vary widely between individual drugs depending on differences in dose, dosing regimen, duration of treatment, pharmacokinetics or mechanisms of actions. Besides sedation, other CNS adverse effects such as aggression, paranoia, social withdrawal or lack of motivation may disrupt or prevent the initiation of normal performance, thus imposing a burden on the ability of the patients to function in a normal manner. Emotional disturbances are rare as indicated by the small number of case reports that mention their existence. Yet theses disturbances sometimes involve severe reactions that are more debilitating than sedation. Behavioural toxicity can be minimised by avoidance of pharmacodynamic and pharmacokinetic drug interactions, adjustment of dosage regimens to a patient's individual response to a drug, nocturnal administration of drugs that are expected to produce sedation and patient education on the potential risks of the drugs they receive. Much of this information can be gained from experimental literature comparing the effect of individual drugs on performance. Unfortunately this is presently incomplete, since most research on behavioural toxicity has been confined to psychiatric drugs. Yet, in the interest of the patient, it should be the responsibility of drug manufacturers and regulators to always identify problematic drugs.