Amitriptyline (Elavil) Causes More Weight Gain Than Mirtazapine (Remeron)
Amitriptyline is associated with greater weight gain (1.8 kg) compared to mirtazapine (1.5 kg), making mirtazapine the slightly better choice if weight gain is your primary concern 1. However, both medications cause clinically significant weight gain and should be avoided if weight-neutral alternatives are appropriate for the patient's condition.
Quantitative Weight Gain Comparison
The 2015 Endocrine Society Clinical Practice Guideline provides the most robust quantitative data from systematic review:
- Amitriptyline: 1.8 kg average weight gain
- Mirtazapine: 1.5 kg average weight gain
This represents a modest 0.3 kg difference between the two agents 1.
Clinical Context and Mechanisms
Amitriptyline is identified as the most potent inducer of weight gain among all tricyclic antidepressants 1. Its weight-promoting effects stem from strong histamine H1 receptor antagonism and anticholinergic properties.
Mirtazapine, while also associated with significant weight gain, demonstrates a slightly more favorable profile 2. The FDA label for Remeron (mirtazapine) reports that 7.5% of patients experienced ≥7% body weight gain compared to 0% on placebo, and 8% of patients discontinued treatment specifically due to weight gain 3. In controlled trials, 17% reported increased appetite on mirtazapine versus 2% on placebo 3.
Direct Head-to-Head Evidence
A long-term double-blind study directly comparing these medications found that objectively measured weight gain occurred in 22% of amitriptyline-treated patients versus 13% of mirtazapine-treated patients 4. This head-to-head comparison reinforces that amitriptyline carries higher risk.
Guideline Recommendations
The Endocrine Society explicitly recommends using a shared decision-making process that provides patients with quantitative estimates of expected weight effects when antidepressant therapy is indicated 1. This allows informed choice when multiple therapeutic options exist.
Important Caveats
Both Cause Clinically Meaningful Weight Gain
While mirtazapine causes slightly less weight gain than amitriptyline, the difference is modest (0.3 kg). Both medications should be considered obesogenic and avoided when weight-neutral alternatives are clinically appropriate 1, 2.
Weight-Neutral or Weight-Loss Alternatives
If depression treatment is the indication, consider:
- Bupropion: Consistently causes weight loss (1.3 kg) 1, 2
- Fluoxetine: Weight loss acutely (1.3 kg), weight-neutral long-term 1, 2
- Sertraline: Weight-neutral in maintenance phase 1, 2
Individual Variation
Recent 2024 real-world evidence from 183,118 patients showed considerable individual variation in weight response, though population-level effects remained consistent 5. Some patients may experience more or less weight gain than average.
Long-Term Considerations
Weight gain with both agents tends to continue or persist during long-term treatment 1, 4. The 1998 long-term study showed sustained weight differences over 2 years of treatment 4.
Practical Algorithm
If choosing between only amitriptyline and mirtazapine:
- Select mirtazapine for marginally less weight gain risk (1.5 kg vs 1.8 kg)
- Counsel patient that both cause significant weight gain
- Monitor weight monthly
- Consider discontinuation if ≥5% weight gain occurs
If other options are clinically appropriate:
- Prioritize bupropion for weight loss effect
- Consider fluoxetine or sertraline for weight neutrality
- Reserve amitriptyline/mirtazapine for cases where their specific properties (sedation, appetite stimulation) are therapeutically desired