Inclusion Criteria:

• The subject suffers from a primary diagnosis of Generalized Anxiety Disorder according to Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision (DSM-IV-TR) criteria.
• The subject has a Hamilton Anxiety Scale total score ≥20 at Screening and Baseline.
• The subject has a Hamilton Anxiety Scale score ≥2 on both Item 1 (anxious mood) and Item 2 (tension) at Screening and Baseline.
• The subject has a Montgomery-Åsberg Depression Rating Scale total score ≤16 at Screening and Baseline.
• Male and female subjects of childbearing potential who are sexually active agree to use adequate contraception from Screening throughout the duration of the study and for 1 month after the last dose of study medication.

Exclusion Criteria:

• The subject has received any investigational compound <30 days before Screening or 5 half-lives prior to Screening.
• The subject has received Lu AA21004 in a previous clinical study or as a therapeutic agent.

• The subject has 1 or more of the following:

  • Any current psychiatric disorder other than Generalized Anxiety Disorder as defined in the DSM-IV-TR.
  • Current or past history of: manic or hypomanic episode, schizophrenia or any other psychotic disorder, including major depression with psychotic features, mental retardation, organic mental disorders, or mental disorders due to a general medical condition as defined in the DSM-IV-TR.
  • Any substance disorder (except nicotine and caffeine) within the previous 6 months as defined in the DSM-IV-TR.
  • Presence or history of a clinically significant neurological disorder (including epilepsy).
  • Neurodegenerative disorder (Alzheimer's disease, Parkinson's disease, multiple sclerosis, Huntington disease, etc).
  • Any Axis II disorder that might compromise the study.

• The subject is required to take excluded medications or it is anticipated that the subject will require treatment with at least 1 of the disallowed concomitant medications during the study including:

  • Nonsteroidal anti-inflammatory drugs
  • Rifampin
  • Macrolide antibiotics
  • Hormones
  • Hypoglycemic agents and Insulin
  • Systemic steroids
  • Antineoplastics
  • Antiobesity agents
  • Antidiarrheal agents (episodic use allowed)
  • Antifungal agents (episodic topical use allowed)
  • Antihistamines (episodic use of loratadine, desloratadine, cetirizine allowed)
  • Cough/cold agents (episodic use allowed but preparations containing pseudoephedrine and narcotics are NOT allowed)
  • Diuretics (episodic use allowed)
• The subject has a significant risk of suicide according to the investigator's opinion or has a score ≥5 on item 10 (suicidal thoughts) of the Montgomery-Åsberg Depression Rating Scale or has made a suicide attempt in the previous 6 months.
• The subject has previously failed to respond to adequate treatment with selective serotonin reuptake inhibitors and/or serotonin-norepinephrine reuptake inhibitors.
• The subject has received electroconvulsive therapy within 6 months prior to Screening.
• The subject is currently receiving formal cognitive or behavioral therapy, systematic psychotherapy, or plans to initiate such therapy during the study.
• The subject has a clinically significant unstable illness, for example, hepatic impairment or renal insufficiency, or a cardiovascular, pulmonary, gastrointestinal, endocrine, neurological, rheumatologic, immunologic, infectious, skin and subcutaneous tissue disorders, or metabolic disturbance.
• The subject has an alanine aminotransferase, aspartate aminotransferase or bilirubin level >1.5 times the upper limits of normal.
• The subject has a serum creatinine of >1.5 times the upper limits of normal.
• The subject has a previous history of cancer that had been in remission for less than 5 years prior to the first dose of study medication. This criterion does not include those subjects with basal cell or stage I squamous cell carcinoma of the skin.
• The subject has clinically significant abnormal vital signs as determined by the investigator.
• The subject has 1 or more laboratory values outside the normal range, based on the blood or urine samples taken at Screening, that are considered by the investigator to be clinically significant.
• The subject has thyroid stimulating hormone value outside the normal range at Screening and is deemed clinically significant by the investigator.
• The subject has an abnormal electrocardiogram as determined by the central reader and confirmed as clinically significant by the investigator.
• The subject has a disease or takes medication that, in the opinion of the investigator, could interfere with the assessments of safety, tolerability, or efficacy.
• The subject has previously participated in this study.