Expected Weight Gain with Mirtazapine at 4 Weeks
When using mirtazapine as an appetite stimulant, you should expect approximately 2-4 kg (4-9 lbs) of weight gain over 4 weeks, though individual variation is substantial.
Weight Gain Incidence and Magnitude
The available evidence provides limited specific quantification for 4-week weight gain, but the following data inform expectations:
Weight gain occurs in approximately 10% of patients (versus 1% with placebo) in clinical trials, making it one of the most common side effects 1, 2.
Mirtazapine causes more weight gain than most other antidepressants, including sertraline, trazodone, and venlafaxine 1.
Increased appetite occurs in about 11% of patients (versus 2% with placebo), which directly contributes to weight gain 1.
Dosing Considerations for Appetite Stimulation
The typical dose range is 15-30 mg/day when used for appetite stimulation, with pilot studies showing efficacy at these doses 3.
Lower doses (15 mg) may produce more sedation and appetite stimulation due to predominant antihistaminic (H1) effects, while higher doses show relatively less somnolence 4.
The recommended starting dosage is 15 mg/day for 4 days, then 30 mg/day, which aligns with the timeframe for appetite effects 2.
Timeline and Mechanism
Appetite stimulation typically begins within the first week of treatment, preceding the antidepressant effects which take 2-4 weeks 5, 4.
Weight gain is attributed to antihistaminic (H1) activity at low doses, causing hyperphagia (increased eating) 4.
Steady-state plasma concentrations are reached in 4-6 days, meaning full pharmacologic effects manifest within your 4-week timeframe 6.
Important Clinical Caveats
This weight gain can be therapeutically beneficial in patients with anorexia, weight loss, or elderly patients with dementia and depression 1.
Tolerance is limited by somnolence and weight gain as the primary side effects, occurring in 23% and 10% of patients respectively 3.
Individual variation is substantial—some patients gain significantly more weight while others gain minimal amounts, making patient counseling about this variability essential 2, 7.
The sedative effects may be beneficial when dosed at bedtime, particularly in patients with concurrent insomnia 1, 5.
Monitoring Approach
Weigh patients at baseline and weekly during the first 4 weeks to track response and adjust dosing accordingly.
Monitor for excessive sedation, which occurs in 19% of patients (versus 5% with placebo) and may limit tolerability 2.
Consider dose adjustment after 10 days if appetite stimulation is insufficient, increasing from 30 mg to 45 mg daily 2.