The most common long-term side effect of mirtazapine is weight gain, though the amount is often smaller than people expect. In premarketing studies, 8% of patients on mirtazapine discontinued the drug specifically because of weight gain. Beyond that, long-term use can affect metabolism, sleep patterns, liver enzymes, and, in rare cases, blood cell counts. Here’s what each of those looks like in practice.
Weight Gain Over Time
Weight gain is the side effect most strongly associated with long-term mirtazapine use, and it’s the reason it gets a reputation as a “fattening” antidepressant. But the actual numbers are more modest than many people assume. On average, patients gain about 1.4 kg (roughly 3 pounds) over 40 weeks of continued therapy. Most of this gain happens in the first 8 to 12 weeks and then levels off.
That said, averages can be misleading. Some people gain significantly more, while others notice no change at all. The weight gain is driven largely by increased appetite, particularly cravings for carbohydrates. Mirtazapine blocks certain receptors involved in satiety signaling, which can make you feel hungrier than usual, especially in the evening. If you’re tracking your weight and notice a steady upward trend beyond the first few months, that’s worth discussing with your prescriber.
Metabolic Changes
Beyond the number on the scale, mirtazapine can subtly shift some metabolic markers. Research in healthy men found that the drug decreased total cholesterol, HDL (“good”) cholesterol, and LDL (“bad”) cholesterol, while the ratio of triglycerides to HDL increased. That ratio matters because a higher value is linked to insulin resistance and cardiovascular risk over time.
There’s also a small signal for blood sugar changes. Glycosylated hemoglobin (a measure of average blood sugar over the previous few months) increased slightly in study participants. Other glucose markers like fasting blood sugar and insulin levels stayed stable, so the clinical significance isn’t entirely clear. If you’re already at risk for type 2 diabetes or have metabolic syndrome, periodic blood work while on mirtazapine is a reasonable precaution.
Sedation and Sleep Effects
Mirtazapine is well known for causing drowsiness, which is why many prescribers recommend taking it at bedtime. In studies, over half of patients reported daytime sleepiness, compared to 18% on placebo. The good news is that most people develop tolerance to this sedating effect within the first week or two of treatment. By the end of week two in one study, daytime drowsiness complaints had largely resolved even as the dose was increased.
For long-term users, the sleep-related picture is actually favorable in one important way: mirtazapine improves sleep continuity (meaning fewer middle-of-the-night awakenings) without disrupting the normal stages of sleep. Unlike many other antidepressants, it does not suppress REM sleep, the phase tied to dreaming and memory processing. This makes it a better option for people whose depression involves significant insomnia. Some residual morning grogginess can persist, though, especially at lower doses where the sedating properties are proportionally stronger.
Liver Enzyme Elevations
In controlled studies, about 2% of mirtazapine users showed clinically significant elevations in a key liver enzyme called ALT, compared to 0% of those on placebo. This doesn’t mean liver damage is common, but it does mean the drug puts a small additional load on the liver. For most people this is clinically insignificant, but if you have pre-existing liver disease or drink heavily, your prescriber may want to check liver function periodically. The elevations observed in studies were typically mild and reversible.
Bone Density in Some Groups
A five-year study tracking postmenopausal women found that mirtazapine use was associated with accelerated bone loss, but only in women who were already low-weight or who had lost weight during the study period. The effect appeared to follow a dose-response pattern, meaning higher doses or longer use correlated with greater bone loss. For women at a healthy weight, no significant acceleration of bone loss was observed. If you’re postmenopausal, underweight, or have other osteoporosis risk factors, this is worth flagging to your doctor, especially if you’ve been on mirtazapine for several years.
Agranulocytosis: Rare but Serious
The most serious potential risk of long-term mirtazapine use is agranulocytosis, a dangerous drop in white blood cells that leaves the body unable to fight infections. This is genuinely rare, occurring in a very small fraction of users, and most cases are reversible once the drug is stopped. However, some fatal cases have been reported, primarily in patients over 65.
Routine blood monitoring isn’t required for everyone on mirtazapine, but you should be aware of the warning signs: unexplained fever, persistent sore throat, mouth sores, or any infection that seems unusual or won’t clear up. These symptoms warrant immediate medical attention and a blood count check. If caught early, the condition resolves after discontinuing the medication.
Withdrawal After Long-Term Use
Stopping mirtazapine abruptly after months or years of use can trigger a discontinuation syndrome. Common withdrawal symptoms include insomnia (both difficulty falling and staying asleep), morning anxiety, nausea, headaches, panic attacks, and gut disturbances. These aren’t dangerous, but they can be deeply uncomfortable and are sometimes mistaken for a relapse of the underlying depression.
The recommended approach is a gradual taper rather than stopping cold turkey. Current guidance suggests reducing the dose by about 10% of the current dose per month. Because mirtazapine tablets don’t come in a liquid form from the manufacturer, some people dissolve dispersible tablets in water to measure smaller dose reductions. A 15 mg tablet dissolved in 15 mL of water creates a 1 mg/mL suspension that allows precise, gradual decreases. Some people taper even more slowly, reducing by as little as 0.1 mg per day.
If withdrawal symptoms flare during a taper, the standard advice is to return to the previous dose, let symptoms settle, and then resume the taper more slowly. There’s no set timeline that works for everyone. People who have been on mirtazapine for years often need a longer, more cautious taper than those who used it for a few months.