Phentermine's Impact on Treatment in Teens

Inclusion criteria\* include, but are not limited to: * Age ≥ 12 years and \< 18 years at time of consent; * Tanner Staging ≥ 2 at the time of screening; * Obesity (BMI ≥ 95th age- and sex-specific CDC percentile or BMI ≥ 30 kg/m2); * Biological females must agree to use adequate contraception, defined as double barrier methods, stable hormonal contraception plus single barrier method, tubal ligation, or abstinence. Exclusion criteria\* include, but are not limited to: * Contraindications to phentermine in adults such as: * A history of cardiovascular disease (e.g., coronary artery disease, stroke, clinically significant congenital heart disease, clinically significant cardiac arrhythmias, and congestive heart failure); * History of glaucoma; * Current or recent (within 14 days of screening) use of a monoamine oxidase (MAO) inhibitor; * Any previous history of drug dependency or current use of an illicit substance (positive urine drug screen); * Current pregnancy or breastfeeding; * Plans to become pregnant within the study duration; * Known hypersensitivity to sympathomimetic amines; * Current nicotine use or nicotine cessation within 3 months of screening; * Stage 2 hypertension (or greater) or taking any medication to treat hypertension; * Current type 1 or type 2 diabetes mellitus or taking any medication to treat diabetes or prediabetes; * Current or recent (within 3 months of screening) use of prescribed weight loss medication(s)/medically prescribed diets (e.g., low calorie, meal replacement/herbal agents/dietary supplements or weight loss program); * Current or recent (within 3 months of screening) use of other sympathomimetic amines such as stimulants to treat attention- deficit/hyperactive disorder; * Use of chronic systemic glucocorticoid therapy (consecutive use of 3 months or more) or other steroid hormone therapy other than oral contraceptives; * Use of tricyclic antidepressants, lithium, levodopa, or dopamine receptor agonists; * History of bariatric surgery; * History or current diagnosis of schizophrenia, psychosis, bipolar disorder, or mental illness requiring hospitalization within 12 months of screening; * History of suicide attempt within 2 years or self-harm within 3 months of screening; * Current Patient Health Questionnaire-9 (PHQ-9) score of ≥ 10; * Current suicidal ideation type 4 or 5 on Columbia Suicide Severity Rating Scale (C-SSRS); * Current Eating Attitudes Test-26 (EAT-26) score ≥ 20 or any history of anorexia or bulimia. * Current condition or disease interfering with metabolism, such as untreated hypo- or hyperthyroidism, Cushing's syndrome; * Current clinically significant hepatic aspartate transaminase (AST) or alanine transaminase (ALT) \> 3x upper limit of age- and sex-specific normal range or renal disease (creatinine clearance \< 60 mL/minute); hypertriglyceridemia (triglyceride ≥ 400 mg/dL) or syndromic or monogenic obesity; * Any clinically significant abnormalities on a standard 12-lead electrocardiogram at baseline; * Heart rate \> 100 bpm at screening; * Current or recent use of any investigational medication or device or participation in an interventional clinical trial within 30 days of screening; * Any clinically significant medical or psychiatric condition or laboratory abnormality that may increase the risk associated with trial participation, investigational product administration, or interpretation of trial results.