Weight Gain Medications for an 18-Year-Old
There are no FDA-approved medications specifically indicated for weight gain in otherwise healthy 18-year-olds, and current clinical guidelines do not recommend pharmacological interventions for weight gain in this population without an underlying medical condition.
Why Medications Are Not Recommended
The available evidence addresses weight management from the opposite perspective—preventing or treating obesity rather than promoting weight gain in healthy individuals. The guidelines and research provided focus exclusively on:
- Obesity prevention and treatment in adults 18 years and older, including pharmacotherapy with agents like phentermine, GLP-1 agonists (liraglutide, semaglutide), and naltrexone/bupropion 1
- Weight loss interventions through dietary modifications and physical activity 1
- Appetite stimulation in cancer patients with cachexia, where megestrol acetate and corticosteroids are used despite significant risks including thromboembolism (1 in 6 patients) and mortality (1 in 23 patients) 2
Critical Context: When Medications Are Used for Weight Gain
The only evidence-based scenarios where weight gain medications are recommended involve serious underlying conditions:
- Cancer-related cachexia: Megestrol acetate increases appetite in 1 in 4 patients and weight in 1 in 12 patients, but carries a 1 in 6 risk of thromboembolic phenomena 2
- Palliative care settings: Corticosteroids like dexamethasone are restricted to 1-3 weeks due to muscle wasting, insulin resistance, and osteopenia 2
- Psychiatric conditions: Certain antidepressants like mirtazapine or amitriptyline cause weight gain as a side effect, but this is generally considered an adverse outcome to be avoided 3, 4
What Should Be Done Instead
For an otherwise healthy 18-year-old seeking weight gain:
- Nutritional intervention is first-line: Increase caloric intake through nutrient-dense foods, focusing on adequate protein, healthy fats, and complex carbohydrates 1
- Resistance training: Implement strength training 2-3 times weekly to build lean muscle mass rather than just fat tissue 4
- Medical evaluation: Rule out underlying causes of low weight including hyperthyroidism, malabsorption disorders, eating disorders, depression, or other metabolic conditions 2
Common Pitfalls to Avoid
- Do not use appetite stimulants off-label in healthy individuals—the risk-benefit ratio is unfavorable given the serious adverse effects (thromboembolism, metabolic complications, muscle wasting) documented in cancer patients 2
- Avoid antipsychotics for weight gain (like olanzapine) outside of psychiatric indications, as they carry metabolic syndrome risks 2
- Do not prescribe anabolic steroids without legitimate medical indication, as they carry cardiovascular, hepatic, and endocrine risks
The absence of evidence-based pharmacological options for healthy weight gain reflects the reality that medications designed to alter weight carry significant risks and should only be used when the underlying condition justifies those risks.