From the FDA Drug Label
2.1 Recommended Dosage The recommended starting dose of mirtazapine tablets is 15 mg once daily, administered orally, preferably in the evening prior to sleep. If patients do not have an adequate response to the initial 15 mg dose, increase the dose up to a maximum of 45 mg per day Patients with Renal Impairment When compared to subjects with normal renal function, total body clearance of mirtazapine was reduced approximately 30% in renal impaired patients with GFR=11–39 mL/min/1.73 m2
The maximum nightly dose of mirtazapine for an 80-year-old patient with impaired renal function (GFR 37) is 45 mg per day. However, considering the patient's renal impairment, a dose reduction may be necessary due to the reduced clearance of mirtazapine. The exact dose reduction is not specified in the label, so a conservative approach would be to consider a lower dose within the recommended range, but the label does not provide a specific dose for this patient population 1 1.
From the Research
For an 80-year-old patient with moderate renal impairment (GFR 37 mL/min, creatinine 149 μmol/L) weighing 69 kg, the recommended upper limit of mirtazapine would be 30 mg nightly. While the standard maximum dose of mirtazapine is 45 mg daily in younger adults with normal renal function, this should be reduced in elderly patients with renal impairment, as supported by studies such as 2 which highlights the importance of considering renal function in dosing. Start with a lower dose of 7.5-15 mg at bedtime and titrate slowly upward if needed, monitoring for side effects. Dose adjustments are necessary because mirtazapine and its metabolites are eliminated primarily by the kidneys, and reduced renal function leads to decreased clearance and potential accumulation, as noted in 3.
Common side effects to monitor include:
- Sedation
- Increased appetite
- Weight gain
- Dizziness
These side effects may be more pronounced in elderly patients. Regular assessment of renal function, mental status, and orthostatic blood pressure is important during treatment, as emphasized in 4 which discusses the assessment of renal function in the geriatric population. If sedation is excessive at higher doses, paradoxically, a dose increase might sometimes reduce this effect due to the medication's complex pharmacology. It's also worth noting that mirtazapine has a favorable tolerability profile compared to other antidepressants, as shown in 5 and 6, which can be an important consideration in the management of elderly patients with multiple comorbidities.