Study of Droxidopa Treatment in Adults With Attention Deficit Hyperactivity Disorder With Co-administration of Carbidopa

Inclusion Criteria: 1. At the time of consent, are between the ages of 18-55, inclusive. 2. Meet DSM-IV criteria for ADHD as assessed by the Adult ADHD Clinician Diagnostic Scale (ACDS) v1.2. 3. Concomitant Axis I diagnoses that are allowed include social anxiety disorder or dysthymia which does not require treatment. Psychiatric co-morbidities will be diagnosed with the Structured Clinical Interview for DSM-IV Axis Disorders (SCID). 4. Must have a satisfactory medical assessment with no clinically significant abnormalities as determined by medical history, physical exam, electrocardiogram (ECG), and clinical laboratory testing. 5. Must be able to swallow capsules. 6. In the opinion of the investigator, the participant must understand and be able, willing and likely to fully comply with the study procedures and restrictions. 7. Must have given signed and dated informed consent in accordance with Good Clinical Practice (GCP) Guidelines. Exclusion Criteria: 1. Lifetime or present history of bipolar or psychotic disorders, that in the investigator's opinion, interfere with the diagnosis and/or with the conduct of the study. 2. Uncontrolled comorbid major depressive disorder, anxiety disorder or dysthymia. 3. Women of childbearing potential who are not using a medically accepted contraception. 4. Sexually active males whose partner is a woman of child-bearing potential must agree to use condoms for the duration of the study and for 4 weeks after the last dose. 5. Women who are pregnant, breast feeding, or plan to become pregnant during the course of this study. 6. Clinically significant electrocardiogram or laboratory abnormalities at screening that are deemed exclusionary in the opinion of the Principal Investigator. 7. Participants taking any psychotropic medication on a regular basis. Participants will need to be free of all psychotropic medications (one week for psychostimulants, four weeks for all other medications), except for as needed (PRN) benzodiazepines or hypnotics. Allowed psychiatric co-morbidities include social anxiety disorder or dysthymia which does not require treatment. 8. Participants with any concurrent chronic or acute illness or unstable medical condition that could, in the opinion of the study physician, confound the results of safety assessments, increase risk to the participant or lead to difficulty complying with the protocol. Participants who have a history of mental retardation or severe learning disability will be excluded. 9. Participants who in the investigator's opinion meet any of the exclusionary criteria specified on the FDA label of either Droxidopa or carbidopa. 10. Have uncontrolled hypertension, defined as systolic blood pressure \>140 millimeters of mercury (mmHg) and/or diastolic blood pressure \>110 mmHg or use of ≥2 antihypertensive medications. 11. Known or suspected hypersensitivity to the study medication or any of its ingredients. 12. Have in the investigator's opinion any significant cardiac arrhythmia. 13. Any significant systemic, hepatic, cardiac or renal illness. 14. Diabetes mellitus or insipidus. 15. Have a history of closed angle glaucoma. 16. Have a known or suspected current malignancy. Participants with a history of cancer must be symptom- and treatment-free for at least 5 years prior to randomization, with the exception of participants with non-melanoma, non-invasive skin cancers (such as basal cell carcinoma), who should not have had an intervention or recurrence within one year of starting the study. 17. Participants with known gastrointestinal illness or other gastrointestinal disorder that may, in the investigator's opinion, affect the absorption of study drug. 18. In the investigator's opinion, have clinically significant abnormalities on clinical examination or laboratory testing. 19. In the investigator's opinion, are unable to adequately co-operate because of individual or family situation. 20. Are not able or willing to comply with the study requirements for the duration of the study. 21. Have participated in another clinical trial with an investigational agent (including named participant or compassionate use protocol) within 1 month before the start of the study. 22. Previous enrollment in the study.