Mirtazapine is generally well tolerated in the short term, but staying on it for months or years can introduce side effects that weren’t obvious in the first few weeks. The most common long-term concerns are weight gain, changes in blood lipids, and persistent sedation, though rarer issues involving the liver, bones, and blood cell counts also deserve attention.
Weight Gain and Metabolic Changes
Weight gain is the most frequently reported long-term side effect of mirtazapine and one of the top reasons people eventually stop taking it. The drug increases appetite through its action on histamine and serotonin receptors, and many people notice cravings for carbohydrates and sweets in particular. Unlike some medications where weight stabilizes after the first few months, mirtazapine-related weight gain can continue for as long as you take it.
Beyond the number on the scale, mirtazapine can shift your blood lipid profile in ways that matter for heart health. A controlled study in healthy men found that just seven days of mirtazapine at a standard dose raised triglyceride levels and lowered HDL cholesterol (the protective kind), and these changes occurred independently of any weight gain. The triglyceride-to-HDL ratio, a marker doctors use to gauge cardiovascular risk, also worsened. Over months or years, these metabolic shifts could compound, especially if weight gain is also in the picture. If you’re on mirtazapine long term, periodic blood work to check your lipid panel and fasting glucose is a reasonable step.
Sedation and Daytime Drowsiness
Mirtazapine is one of the more sedating antidepressants available, particularly at lower doses (7.5 to 15 mg). This sedation comes from its strong blocking of histamine receptors in the brain. For people with insomnia alongside depression, that drowsiness can actually be helpful at first.
The good news is that many people develop tolerance to this effect relatively quickly. Case reports document the sedative action noticeably fading within the first week or two of use. At higher doses (30 mg and above), additional brain chemistry effects tend to counterbalance the histamine blockade, making drowsiness less pronounced. Still, some people remain sensitive to the sedation for as long as they take the drug, experiencing morning grogginess or difficulty concentrating during the day. If persistent drowsiness is a problem, taking the full dose at bedtime and discussing dose adjustments with your prescriber are the most practical strategies.
Bone Health and Fracture Risk
Long-term mirtazapine use is associated with a modest increase in hip fracture risk. A large observational study found that continuous mirtazapine users had roughly 27% higher odds of hip fracture compared to non-users. That risk climbed dramatically when mirtazapine was combined with other antidepressants, particularly SSRIs or older tricyclic antidepressants, where the odds of fracture increased several-fold.
The mechanism likely involves both the sedation itself (which raises fall risk, especially in older adults) and potential direct effects on bone metabolism that antidepressants as a class can have. If you’re over 65, have osteoporosis, or take other medications that affect balance, this is worth discussing with your doctor. Staying physically active and ensuring adequate calcium and vitamin D intake can help offset some of this risk.
Liver Enzyme Elevation
Up to 10% of people on mirtazapine show some degree of elevated liver enzymes on blood tests. In most cases, these elevations are modest and don’t cause symptoms or require any change in treatment. Rare instances of more serious liver injury, with markedly elevated enzymes and occasionally jaundice (yellowing of the skin or eyes), have been reported but remain uncommon. The National Institutes of Health classifies mirtazapine as a “probable rare cause” of clinically significant liver disease. If you’re on the medication long term, routine liver function tests can catch any concerning trends early.
Blood Cell Count Changes
One of the rarer but more serious long-term risks is a drop in white blood cells, a condition that can range from mild neutropenia to severe agranulocytosis. Agranulocytosis leaves your body unable to fight infections effectively, turning an ordinary sore throat or mouth sore into a potentially dangerous situation. This side effect occurs in fewer than 1 in 10 users and severe cases are rarer still, but it’s the reason prescribers sometimes order periodic complete blood counts during treatment. If you develop unexplained fevers, persistent sore throat, or mouth ulcers while on mirtazapine, getting a blood count checked promptly is important.
Heart Rhythm Effects
Some antidepressants carry meaningful risks for heart rhythm changes, but mirtazapine appears relatively safe in this regard. A study of hospitalized patients found that the average change in QTc interval (a measure of electrical timing in the heart) was essentially zero after starting mirtazapine. A separate six-week controlled trial in people with depression showed only a 1.6-millisecond increase in QTc, which is not clinically significant. No dangerous heart rhythm events were observed in either study. While individual variation exists, mirtazapine at standard doses does not appear to pose a notable cardiac risk for most people.
Discontinuation Can Be Challenging
While not a side effect of taking mirtazapine, the difficulty of stopping it is something every long-term user should understand. Mirtazapine can cause withdrawal symptoms if stopped abruptly, and the brain’s adaptation to the drug means that even modest dose reductions can trigger discomfort. Common withdrawal experiences include dizziness, nausea, irritability, insomnia, and a general flu-like feeling.
Current best practice for tapering involves reducing the dose by about 10% of the current dose each month, rather than making large jumps. This “hyperbolic” approach reflects the fact that the brain’s receptors respond more dramatically to changes at lower doses than at higher ones. Because mirtazapine doesn’t come in a liquid form from manufacturers, people who need precise small reductions can dissolve a dispersible tablet in water (for example, a 15 mg tablet in 15 mL of water to create a 1 mg per mL mixture) or work with a compounding pharmacy. Rushing the taper is the most common cause of unnecessary withdrawal symptoms, so patience during this process pays off.
Putting the Risks in Context
Most people who take mirtazapine long term will deal with weight gain and possibly some lingering drowsiness. The metabolic, liver, bone, and blood count concerns are real but manageable with periodic monitoring. Compared to many other antidepressants, mirtazapine has a relatively favorable cardiovascular profile and tends to cause fewer sexual side effects, which is often why it gets prescribed in the first place. The key to safe long-term use is staying aware of these potential issues, keeping up with routine blood work, and having a plan for eventual tapering if you and your prescriber decide it’s time to stop.