Open-label Study to Evaluate Long-term Safety and Efficacy of SPN-812 (Viloxazine Extended-release Capsule)

Cohort 'A' Inclusion/Exclusion Criteria: Inclusion Criteria 'A': 1. Completion of a previous blinded study of SPN-812 for the treatment of ADHD. 2. Continues to be medically healthy and with clinically normal laboratory profiles, vital signs, and electrocardiograms (ECGs). 3. Weight of at least 20 kg if 6-11 years old and at least 35 kg if 12 years or older. 4. Written Informed Consent obtained from the subject's parent or legally authorized representative (LAR); and written Informed Assent obtained from the subject if appropriate; written Informed Consent obtained from the subject if he/she is 18 years old. 5. Females of Childbearing Potential (FOCP) must be either sexually inactive (abstinent) or, if sexually active, must agree to use one of the following acceptable birth control methods beginning 30 days prior to the first dose, throughout the study: 1. simultaneous use of male condom and intra-uterine contraceptive device placed at least four weeks prior to the first study drug administration 2. surgically sterile male partner 3. simultaneous use of male condom and diaphragm with spermicide 4. established hormonal contraceptive Exclusion Criteria 'A': 1. Current diagnosis of significant systemic disease or of a major psychiatric or neurological disorder, including history or family history of seizures or seizure-like disorders. History of Major Depressive Disorder is allowed if the subject is free of episodes currently and for the last six months. 2. Current evidence of suicidal ideation and/or behaviors (assessed with Columbia Suicide Severity Rating Scale or C-SSRS). 3. Body Mass Index (BMI) greater than 95th percentile for the appropriate age and gender (according to the CDC BMI-for-Age Growth Charts for boys and girls). 4. Pregnancy, breastfeeding, or refusal to practice contraception during the study for female subjects of childbearing potential (FOCP). 5. Current substance or alcohol use. 6. Any reason which, in the opinion of the Investigator, would prevent the subject from participating in the study. Cohort 'B' Inclusion/Exclusion Criteria: Inclusion Criteria 'B': 1. Has completed a previous Study 812P401 and is less than 6 years of age at Visit 1 in this study (812P310). 2. Continues to be medically healthy with clinically normal laboratory profiles, vital signs, and electrocardiograms. 3. Weighs at least ≥5th percentile for age and sex. 4. Parent or LAR is willing and able to provide Written Informed Consent. Exclusion Criteria 'B': 1. Has a current diagnosis of significant systemic disease and/or of a major psychiatric or neurological disorder. 2. Current evidence of suicidal ideation and/or behaviors (assessed with Columbia Suicide Severity Rating Scale or C-SSRS), is at significant risk of suicide, either in the opinion of the Investigator or defined as a "yes" to suicidal ideation C-SSRS questions 4 or 5 or answering "yes" to suicidal behavior on the C-SSRS at Visit 1 or has attempted suicide from day of end of study in previous blinded study to day of Visit 1 in this study (812P310). 3. Has a BMI \>95th percentile for the appropriate age and sex (according to the CDC BMI-for-Age Growth Charts for boys and girls). 4. Any reason which, in the opinion of the Investigator, would prevent the subject from participating in the study.