15. History of substance dependence, alcohol abuse, or excessive alcohol intake (defined as an average weekly intake of > 21 alcoholic drinks for men or > 10 alcoholic drinks for women) within 3 years prior to screening, and/or a positive screen for drugs of abuse or alcohol at screening or on admission to the study unit. Subjects who use tricyclic antidepressants or benzodiazepines for an established clinical indication may be permitted to enter the study based upon the judgement of the investigator.