This study will be a randomized, double-blind, placebo-controlled crossover study to assess the effects of GTS21 (25 mg three times a day (tid), 75 mg tid, 150 mg tid) compared to placebo in non-smoking adults aged 18-55 with a diagnosis of ADHD, any subtype.

Inclusion Criteria:

• Male or female between the ages of 18-55, inclusive.
• Diagnostic and Statistical Manual for the Classification of Mental Disorders, Fourth Edition (DSM-IV) criteria for ADHD, any subtype, based on a detailed psychiatric evaluation including the Conners' Adult ADHD Interview for DSM-IV (CAADID) and the Structured Clinical Interview for DSM-IV (SCID).
• A minimum Total ADHD Symptoms Index score of 28 on the clinician administered CAARS.
• A Clinical Global Impressions-Severity (CGI-S) score of ≥ 4 at Screening.
• Normal or clinically insignificant ECG and clinical laboratory (e.g., liver enzymes, complete blood count, etc.) findings at Screening.
• Intellectual function at age-appropriate levels, as deemed by the Investigator.
• Supine systolic and diastolic blood pressure measurements < 140 and < 90, respectively, at Screening.
• Written, signed and dated informed consent for the patient to participate in the study must have been given by the patient.
• Females of child-bearing potential must have had a negative serum beta human chorionic gonadotropin (HCG) pregnancy test at Screening and be practicing double-barrier methods of contraception, if sexually active and for 30 days following administration of any study drug.
• Male patients who were sexually active must have agreed to use a reliable form of contraception during the study and for 30 days following administration of any study drug.
• Be fluent in English (speaking, writing and reading).

Exclusion Criteria:

• Any current, controlled (requiring a prohibited medication) or uncontrolled, comorbid psychiatric diagnosis (except simple phobias), all major depressive disorders [dysthymia and mood disorder not otherwise specified (NOS) allowed unless medication required],and any severe comorbid Axis II disorders or severe Axis I disorders such as Post Traumatic Stress Disorder, bipolar illness, psychosis, obsessive-compulsive disorder, substance abuse disorder, or other symptomatic manifestations that, in the opinion of the Investigator, contraindicated treatment with GTS21 or confound efficacy or safety assessments.
• Any condition or illness (including clinically significant abnormal laboratory values) which, in the opinion of the Investigator, represented an inappropriate risk to the patient and/or could confound the interpretation of the study.
• Regular use of nicotine products (including 90 days before Screening), including smoking, transdermal patch, chewing tobacco, etc. (verified via salivary cotinine levels at Screening).
• Current use of any prohibited medication or other medications, including herbal supplements, that have central nervous system (CNS) effects or affect cognitive performance, such as sedating antihistamines and decongestant sympathomimetics (bronchodilators were permitted).
• Use of another investigational product or participation in a clinical study within 30 days prior to Screening.
• Body Mass Index (BMI) > 32.
• Known or suspected allergy, hypersensitivity, or clinically significant intolerance to nicotine or nicotinic agonists.
• Clinically important abnormality on urine drug screen (excluding the patient's current ADHD stimulant, if applicable) at Screening.
• Pregnant or currently lactating.
• Patients that had previously been enrolled into this study and subsequently withdrawn.