Long-term use can bring ongoing weight gain, appetite increase, sleepiness, dry mouth, constipation, and withdrawal symptoms if the dose drops too fast.
Mirtazapine can be a good fit for some people, especially when poor sleep and low appetite sit alongside depression. But the same traits that make it useful can also be the ones that stick around. The long-term side effects people ask about most often are weight gain, feeling sleepy, dry mouth, constipation, and the rough patch that can hit if the medicine is cut down too fast.
That does not mean everyone taking it for months or years will run into trouble. Many people do fine on it. The bigger issue is knowing which problems tend to hang on, which ones often settle, and which ones call for a quicker medication review.
How Long-Term Effects Usually Show Up
When people say “long-term side effects,” they usually mean one of two things. The first is a side effect that keeps showing up while the drug is still being taken. The second is a problem that appears during dose cuts or after stopping.
With mirtazapine, the day-to-day issues are often plain: you may feel hungrier, sleepier, slower in the morning, or backed up. Over time, those can matter more than they did in week one because they affect weight, routines, driving, work, and sleep quality.
Long-Term Side Effects Of Mirtazapine That People Notice Most
The FDA prescribing information lists somnolence, increased appetite, weight gain, dizziness, dry mouth, and constipation among the reactions seen in trials. Those numbers came from short controlled studies, so they do not predict exactly what one person will feel after six months or a year. Still, they give a solid starting point for the side effects most likely to linger.
Weight Gain And Appetite Increase
This is the long-haul issue many people notice first. Mirtazapine can make food more appealing and hunger harder to ignore. In the FDA label, increased appetite was reported in 17% of patients in six-week trials, and weight gain in 12%. In a separate pooled set of studies, 7.5% of patients had body-weight increases of at least 7%.
Weight gain tends to matter more with time because it can creep up in small steps. A few pounds may not feel like much in month one. Over several months, it can change how clothes fit, how you sleep, and how willing you are to stay on the drug.
Sleepiness That Sticks Into The Day
Mirtazapine is well known for making people drowsy. In FDA trials, somnolence was reported in 54% of patients, and it led some people to stop treatment. For some, that sleepy effect eases after the body gets used to the drug. For others, the heavy, foggy feeling hangs on, especially in the morning.
If you feel slowed down the next day, this side effect is not just annoying. It can affect driving, work that needs clear thinking, and the simple act of getting out of bed on time.
Dry Mouth And Constipation
These two can stay in the background for a long time. Dry mouth may sound minor, yet a constant dry, sticky feeling can make eating less pleasant, raise thirst, and bother your teeth and gums. Constipation can do the same slow damage to daily comfort, with bloating, cramping, and strain.
In the FDA trials, dry mouth showed up in 25% of patients and constipation in 13%. Those are not tiny numbers, and both can linger if nothing is changed around the dose, timing, fluids, fibre intake, or other medicines.
| Side effect | What it can feel like over time | What the trial data showed |
|---|---|---|
| Sleepiness | Morning grogginess, slower thinking, trouble driving or working early in the day | 54% with mirtazapine vs 18% with placebo |
| Increased appetite | Feeling hungry more often, larger portions, more snacking | 17% vs 2% |
| Weight gain | Gradual rise on the scale, tighter clothes, lower willingness to stay on treatment | 12% vs 2%; 7.5% had at least 7% body-weight gain in pooled studies |
| Dry mouth | Sticky mouth, thirst, trouble with dental comfort | 25% vs 15% |
| Constipation | Bloating, hard stools, abdominal discomfort | 13% vs 7% |
| Dizziness | Light-headed spells, unsteady moments, worse mornings for some people | 7% vs 3% |
| Cholesterol rise | No daily sensation in many cases, found on blood work | 15% had nonfasting cholesterol rises above 20% over the upper limit of normal vs 7% |
| Triglyceride rise | Often silent, found on blood work | 6% reached at least 500 mg/dL vs 3% |
What Often Eases And What May Linger
There is some good news here. On the NHS common questions page for mirtazapine, the medicine is described as generally safe for long-term use, with no clear lasting harmful effects seen over many months or years. That point matters. Long-term side effects are usually about burden while taking the drug, not proof of permanent damage.
Still, “generally safe” does not mean “easy for everyone.” Weight gain can build with time. Sleepiness can stay bothersome if your body never fully adjusts. Dry mouth and constipation can settle into the daily routine and wear you down by inches. The pattern is less about one dramatic moment and more about small problems that keep showing up.
Problems That Need A Closer Watch
Some issues are less common but carry more weight when they show up. The FDA label notes rises in cholesterol and triglycerides, low sodium levels, liver enzyme changes, angle-closure glaucoma in susceptible people, and heart-rhythm problems in people who already have risk factors. These are not the side effects most people feel day to day, yet they are part of the longer-term safety picture.
The NIH’s MedlinePlus drug monograph also lists drowsiness, dizziness, and increased weight and appetite, then flags urgent symptoms such as fever, sore throat, confusion, chest pain, unusual bleeding, seizures, or vision changes. Those are the symptoms that should not be brushed off as “just the medicine.”
Stopping Too Fast Can Feel Like A Side Effect
One reason people get mixed up about long-term side effects is that withdrawal can blur the picture. A person may have been doing fairly well on mirtazapine, then feel much worse when the dose is cut too fast. The FDA label lists dizziness, abnormal dreams, strange skin sensations, agitation, anxiety, fatigue, confusion, headache, tremor, nausea, vomiting, and sweating among the problems reported during abrupt stopping.
That is why the dose is usually reduced step by step instead of all at once. If symptoms flare during a taper, it does not always mean the original condition is back. Sometimes it means the nervous system needs a slower landing.
| Issue | Usual pattern | Next step |
|---|---|---|
| Weight gain or rising appetite | Builds over weeks or months | Bring it up at a routine medication review, especially if the trend keeps climbing |
| Daytime sleepiness | May ease, but can stay | Ask about dose timing, dose size, alcohol use, and other sedating drugs |
| Dry mouth or constipation | Can linger quietly for months | Review fluids, food pattern, bowel changes, and whether another drug is adding to it |
| Dizziness during dose cuts | Often tied to taper speed | Ask whether the taper needs to slow down |
| Fever, sore throat, chest pain, seizure, severe confusion, or vision change | Not a “wait and see” problem | Get urgent medical help |
What Raises The Odds Of Trouble
A few patterns show up again and again:
- Taking mirtazapine with alcohol or other sedating drugs can make sleepiness worse.
- People who already struggle with appetite or weight may notice the change sooner.
- Older adults can be more prone to low sodium and falls tied to dizziness or weakness.
- People with heart-rhythm risk factors need more care around QT-related warnings.
- Anyone stopping after long use may need a slower taper than they expected.
When A Medication Review Makes Sense
It is worth checking in with your prescriber if the medicine is helping your mood but the side effects are chipping away at daily life. A review also makes sense if your weight keeps rising, you are too sleepy to drive safely, constipation is turning into a regular problem, or you want to stop and need a taper plan.
The big picture is fairly steady across major sources: mirtazapine is often safe to use long term, but the side effects that tend to last are weight and appetite changes, sedation, dry mouth, constipation, and dose-reduction symptoms. If you know that ahead of time, the drug is a lot less likely to catch you off guard.
References & Sources
- Food and Drug Administration (FDA).“REMERON (mirtazapine) tablets, for oral use.”Lists common adverse reactions, discontinuation symptoms, and warnings on cholesterol, triglycerides, sodium, heart rhythm, and eye risk.
- NHS.“Common questions about mirtazapine.”States that mirtazapine is generally safe for long-term use and outlines withdrawal symptoms and weight gain concerns.
- MedlinePlus.“Mirtazapine: Drug Information.”Summarizes common side effects and the urgent symptoms that call for prompt medical care.