Mirtazapine | Antidepressant Uses & Side Effects

Medication name

Mirtazapine ("mir-TAZ-a-peen")
Brand name

Zispin ("ZISS-pin")
Medication type

Tetracyclic antidepressant

Tablets: 15mg, 30mg and 45mg strengths

Orodispersible (‘melt in your mouth’) tablets: 15mg, 30mg and 45mg strengths

Liquid: 15mg in 1ml (sugar-free)

If you are 18 or over, the doctor can prescribe mirtazapine for you as a licensed medicine for depression (low mood).

If you are under 18, a specialist doctor may still consider this as a treatment option. Your doctor may also consider this medicine for anxiety.

Read our guide to depression

About mirtazapine

Mirtazapine is a tetracyclic antidepressant. It works by adjusting the levels of noradrenaline and serotonin in your brain.

Serotonin (also called '5HT') and noradrenaline are a naturally occurring chemical messengers (or ‘neurotransmitters’) that have an important role in areas of the brain that control mood, thinking, feelings and emotions.

When you’re depressed, these chemical messengers are not as effective or active as usual. Mirtazapine increases the amount of these chemical messengers in the brain, which can help to improve mood.

Depression, and other conditions like anxiety disorders, are not just caused by low serotonin levels, but a rise in serotonin levels and other neurotransmitters can improve symptoms and help you to feel better.

Mirtazapine and everyday life

Mirtazapine can start to work on depression within the first one to two weeks of treatment, and the improvement continues over the following few weeks. It may take four weeks or longer for you to feel the full effect.

Your doctor might start you on a low dose and then increase it slowly over a few weeks to your full dose.

How people respond to treatment can vary. Sometimes improvement is slow, and you may not feel any different to start with. This can be hard when your mood is low, and if you experience any side effects from your medication you may think you feel worse and not better. If you can, give your medication a chance to work and continue to take it for at least three to four weeks to see if it makes a difference.

Your parents, friends and doctor may notice an improvement in you before you feel it yourself, so it’s a good idea to talk to them.

If you think your medicine has not made any difference to how you feel after three to four weeks, you should go back to your doctor. They might recommend a change in dose or a different medication. However, it can take longer to work for some people than for others, so they may suggest you wait six to eight weeks before deciding.

A potential side effect of mirtazapine is an increase in appetite and weight gain. This happens more often in young people than in adults.

It is not possible to predict how mirtazapine will affect each person before they start taking it. Talk to your doctor or pharmacist if you are concerned about your weight, or changes to your weight, while taking mirtazapine.

A very common side effect of mirtazapine is a dry mouth. Over a long time, this can increase your risk of developing tooth decay or gum disease. Make sure you brush your teeth well and have regular dental check-ups. Speak to your dentist, doctor or pharmacist about things you can do to help.

You may want to let your family and friends know you are taking mirtazapine so they can support you and help you look out for side effects.

For guidance on this, check out our page on getting support with your medication.

Mirtazapine can make you feel very sleepy, especially when you first start taking it. This should improve within a few weeks. If it makes you feel drowsy, try taking it just before you go to bed.

Some people can get a side effect where it makes it difficult to get to sleep.

If you have been taking mirtazapine for more than a month and you continue to feel very tired, sleepy or struggle to function during the day, you should go back to your doctor and discuss what changes could be made to help. This may include switching to a different medicine.

Alcohol

It may be possible to drink some alcohol while taking mirtazapine but having the two together might make you very sleepy and unsteady on your feet.

During the first few weeks of treatment, it is recommended that you don’t drink alcohol until you know how the medicine affects you.

Drinking alcohol every day, or in large amounts, can make your symptoms worse and may mean you won’t get the maximum benefit from your medication.

Street drugs

Cannabis can make drowsiness worse with mirtazapine and give you a fast heartbeat.

Cannabis and other drugs may have their own side effects on your mental health, like anxiety or psychosis.

Methadone or heroin can make drowsiness worse with mirtazapine.

Mirtazapine could raise the level of cocaine in your body, giving you a bigger reaction.

Taking mirtazapine with cocaine or ecstasy is likely to dampen the stimulant effects. Ecstasy may also increase your levels of mirtazapine as it interferes with its breakdown in the liver.

You should not mix mirtazapine and amfetamines unless you are under close medical supervision, as an increased amfetamine response may be dangerous.

There are many other street drugs, but we don’t know what effect taking them with mirtazapine will have, so it’s best to be cautious. There is no regulation of street drugs or ‘legal highs’, so even if there are no known issues with the medication you take, the supply you receive might be mixed with other substances that could be dangerous.

Smoking

Cigarette smoke affects the amount of mirtazapine in your body.

If you smoke, you may need a higher dose of mirtazapine than someone who does not smoke.

Tell your doctor if you smoke, so that you get the right dose for you.

If you stop smoking, the body’s mirtazapine level rises, and you might need to reduce your dose of mirtazapine slowly over one week.

If you start or restart smoking, you will probably need to increase your dose of mirtazapine again.

Go to your doctor for advice if you stop or start smoking, or if you switch to vaping or a nicotine replacement product instead of smoking.

Get more advice on our drugs and alcohol guide.

Mirtazapine does not mix well with some other medicines, including herbal remedies. It may affect how they work or could cause more side effects.

Do not take mirtazapine if you take monoamine oxidase inhibitor antidepressants (MAOIs) or have taken them in the last two weeks. MAOIs include isocarboxazid, moclobemide, phenelzine and tranylcypromine.

Before you start taking mirtazapine, tell your doctor if you are taking any other medications including things you have bought from a pharmacy or other shop for common illnesses like colds and flu or medications that you put on your skin.

Always tell the pharmacist that you are taking mirtazapine if you buy any medicines from a pharmacy.

Look at the leaflet inside your medicine box for more information about other medicines that can interact with mirtazapine. With some medicines your doctor may need to adjust the dose of your medicines if you take them together. Ask a doctor or pharmacist for more information if you have any questions.

The mirtazapine tablets that you swallow may not be suitable for you if you have problems eating some sugars or dairy foods, as they contain lactose.

The mirtazapine orodispersible (‘melt in your mouth’) tablets contain aspartame, which can be a problem for people who have a condition called phenylketonuria.

The oral solution contains maltitol, which can be a problem for anyone who has an intolerance to a sugar called fructose.

There are several companies that manufacture this medicine and the non-active ingredients may vary between products.

Always let your pharmacist know if you have any food allergies or intolerances, and always check with them if you’re concerned about any of the ingredients in your medication.

If you need to avoid animal products, please note that non-active ingredients used in the production of medicines may sometimes be of animal origin. Ask your pharmacist if you have any questions about the ingredients.

Further information about practical considerations for medicines if you need to avoid animal products can be found on the Vegan Society website.

Taking mirtazapine may make you feel tired, dizzy or less alert when you start taking it.

This could affect you if you drive a car, ride a bike, use machines, or do anything else that needs a lot of focus.

It might be best to stop doing these things for the first few days or weeks until you know how it affects you or the side effects pass.

Most people drive as normal while taking mirtazapine. If you are worried about this or have any concerns you would like to discuss, speak to your doctor or pharmacist.

Pregnancy

If you become pregnant while you are on mirtazapine, you should carry on taking it and make an appointment to see your doctor as soon as possible They can discuss the benefits and risks of continuing mirtazapine and help you to make a decision that is best for you and your baby.

They may refer you to a specialist perinatal mental health team to support you with this decision.

You can find out more information about taking amitriptyline during pregnancy at Bumps (Best Use of Medicines in Pregnancy).

Untreated depression can also be harmful to you and your developing baby, so decisions about stopping or avoiding medication when you are pregnant need to be discussed carefully with your doctor. If you and your doctor agree that it is best for you to continue taking mirtazapine while pregnant, you should tell your midwife. If you are planning to get pregnant, it is recommended that you take folic acid while you are trying for a baby and during pregnancy. It is safe to take this together with mirtazapine.

Post-natal

With some anti-depressants taken in the later stages of pregnancy, there is evidence that there is an increased risk of particular health conditions. There are no studies that show mirtazapine increases this risk, but it cannot be ruled out.

Some newborn babies might experience symptoms at birth or soon after if the mothers take mirtazapine in the last three months of pregnancy like feeding problems or restlessness.

Tell your midwife if you are taking mirtazapine, so that they can help if the baby has any symptoms after birth.

Breastfeeding

Mirtazapine is passed to the baby in breastmilk in small amounts.
If your baby was born early, then breastfeeding while you are taking mirtazapine may not be recommended. Talk to your midwife and doctor about feeding options.

If you breastfeed while taking mirtazapine, seek urgent medical advice if your baby becomes restless, very sleepy or developing feeding problems.

Sex

Antidepressants, such as mirtazapine can have side effects that affect your sex life. These include:

Painful erections or difficulty getting an erection.
Bleeding from the vagina.
Problems with reaching orgasm.
Reduced genital sensation.
Lower sex drive.
A long-lasting and painful erection (priapism) that continues for more than a few hours. This is a very rare side effect but if this happens, you need to go to hospital for treatment.

Untreated depression can have a negative effect on your sex life, so if mirtazapine helps you to recover, positive effects can include improving your desire, experience and enjoyment of sex as your mood lifts and you become interested in life and relationships again.

Fertility

There is no evidence to show that mirtazapine affects human fertility. Talk to your doctor about your mirtazapine if you are trying to get pregnant or planning a family.

Mirtazapine is not a banned substance in sport.

Most people play sports as normal while taking mirtazapine, but if mirtazapine affects your concentration, eyesight or co-ordination, then you may want to wait to see if those effects go away before playing sports that need a lot of focus. If you have concerns, discuss this with your doctor.

Mirtazapine may affect your concentration, make you feel tired or dizzy or less alert.

Ideally it is best not to take mirtazapine for the first time just before your exams.

It is not possible to predict how the medication will make you feel, and if you do get any side effects, these are more likely to occur when you first start taking mirtazapine.

You should talk to your doctor about any future exams if you are starting mirtazapine. You might decide together to delay starting it until you have done them.

If they are more than a month away, however, you might find that it is better to start mirtazapine to improve your motivation and ability to study.

Most people take exams as normal while taking mirtazapine, but if you have any concerns, discuss this with your doctor.

Information and safety considerations

Your doctor will consider any other medical conditions or symptoms you have before recommending a medicine for you.

Your medicine will come with a ‘patient information leaflet’. It’s important that you read this information. Speak to your pharmacist if:

you’re not given a leaflet
you don’t understand the information
you need it in a different format or language
you’re concerned about something you’ve read

The information leaflet also includes a list of warnings and precautions to consider before you take the medicine. If you think that any of these apply to you and are concerned that your doctor is not already aware of them, check with your doctor or pharmacist before you start to take your medication (or as soon as possible if you are already taking it).

Uses, warnings, safety and side effects

Taking mirtazapine

Your doctor will advise you on how long it is best to keep taking your medication. This may vary depending on what you’re taking it for and how many times you’ve been unwell.

If you are prescribed mirtazapine for low mood or depression, it is recommended that you take it for at least six months after you feel well again. This means that most people take it for at least seven to nine months. Research has shown that stopping it sooner, or immediately after you start to feel better, increases the risk of your low mood returning.

If you have had low mood or depression more than once, or if there is a higher risk of you becoming unwell again, your doctor may advise you to continue mirtazapine for at least two years after you feel well again.

If you are prescribed mirtazapine for anxiety, it is recommended that you take mirtazapine for at least one year after you feel well again. Research has shown that stopping it sooner, or immediately after you start to feel better, increases the risk of your symptoms returning.

You will get the best effect from mirtazapine if you take it every day at the dose prescribed by your doctor.

Mirtazapine should usually be taken once a day. It is best to take it at bedtime because it can make you feel sleepy.

You can take your medicine before or after food.

For the plain tablets, swallow them whole with a drink of water or juice – if you chew them, they taste bitter. The plain tablets should also be swallowed whole with at least half a glass of water while you are sitting up or standing up. This is to make sure that they reach the stomach and do not stick in your throat.

For the orodispersible tablets, peel off the cover from the blister pack rather than pushing the tablet through, so it does not break. Then put the tablet on your tongue and let it dissolve there. It can be swallowed without water.

The oral solution should come with a measuring syringe to help you get the dose right. Read the instructions before you use it.
You can take the dose directly from the measuring syringe or add it to a glass of water to take immediately.

If you forget to take a dose, then just take it as soon as possible after you remember.

If you forget your bedtime dose of mirtazapine and you remember in the morning, you may wish to miss that dose if your medicine makes you very sleepy. Just take your next dose at bedtime as usual. However, if you are regularly missing doses of your medication this will reduce the beneficial effect of your medicine.

Do not try to catch up by taking a double dose because this could cause you to have extra side effects.

Try to get into a routine, like taking it when you brush your teeth, to help you remember to take the dose at about the same time every day.

If you forget to take it for a few days, you may start getting withdrawal symptoms such as headache, dizziness, anxiety, or feeling sick. Withdrawal symptoms should improve quickly after you start taking mirtazapine again. If they do not, seek advice from your doctor or pharmacist.

If you regularly forget to take it every day, your medicine may not work as well and it could increase the risk of you becoming unwell again.

If you have been taking mirtazapine regularly for more than one to two months, it is recommended that you stop gradually (ideally over at least four weeks).

Discuss stopping your medication with your doctor. If you have important events coming up or going through a period of life which is very demanding, it might not be the best time to stop your medication. Planning and support are important to stopping your antidepressant medication successfully.

Antidepressants are not addictive, but if stopped suddenly they can cause discontinuation or withdrawal symptoms. These are caused by the receptors and levels of chemical transmitters in your brain readjusting after you reduce or stop your medication (and can also occur if you miss doses of medication).

Some of the symptoms you get may be like symptoms you had when you were unwell, so understandably, you might feel anxious or upset that your illness could be returning.

If you experience symptoms immediately or soon after stopping medication (days rather than weeks), this is usually a withdrawal symptom caused by your body adjusting to not having the medication anymore. If the symptoms start after a month or more after stopping your medicine, this might be a sign that you are becoming unwell again.

Withdrawal symptoms usually start within a few days of stopping (or sometimes reducing) your medication. A wide range of symptoms have been reported. You may experience a few or a range of these:

dizziness and/or feeling unsteady
headaches
numbness or tingling in hands or feet
sleep disturbances (vivid dreams, nightmares, not being able to sleep)
electric shock-like feelings in the head, spine, arms, legs or hands
feeling anxious, agitated or irritable and/or restless
confused or disorientated and reduced concentration
poor appetite, feeling or being sick or having diarrhoea (loose poo)
sweating or shaking
feeling emotional, low mood and tearful
flu-like symptoms/muscle aches and pains
tinnitus (ringing in your ears)
problems with your eyes, or fluttering/pounding heartbeat (this is not as common as the other symptoms listed above)

It is not possible to predict if you will experience withdrawal symptoms (or how bad they will be if you do experience them) but studies report about half of people may get them. The risk appears to be higher if:

you have been taking your medication for six to eight weeks and you stop taking it suddenly
you are a child or young person
you experienced an increase in anxiety symptoms when you started medication
you have experienced withdrawal symptoms with another antidepressant before

If you do experience symptoms, they are usually mild and don’t last long (two to three weeks for most people), but for a few these could be severe enough to make you feel very unwell and last longer (two to three months or more). Arrange an appointment with your doctor to discuss your symptoms and seek advice if you are concerned or unable to tolerate them.

You can stop taking mirtazapine safely with your doctor’s help. Planning a gradual withdrawal together can help to minimise or prevent the withdrawal symptoms you experience.

Warnings and safety

If you have taken more than the dose prescribed by your doctor, contact NHS 111 immediately to ask for advice, or ask a family member or friend to do this for you. This is important even if you don’t feel any different.

NHS 111 can give you advice and direct you to the best place to get more help if you need it.

If you have taken too much medication on purpose, or if after taking this you quickly feel unwell, call 999 or get someone to take you to A&E straight away. This includes any of the following signs:

feeling sleepy or disorientated
having a fast heartbeat

If you need to go to A&E, do not travel alone or drive yourself there. Get your parent, family member or friend to go with you to support you and keep you safe on the way. If someone is not able to drive you there, call for an ambulance. Take your medication with you and tell doctors how much you have taken.

Suicidal thoughts

Depression (and other mental illnesses) can sometimes cause you to have thoughts about hurting yourself or taking your own life. While uncommon, this could also be a side effect of your medicine. This can happen to anyone but is more likely to happen if you are under 25 years old.

If you have any thoughts like this, it is important you get urgent help. Contact your doctor immediately for advice and support. This could be your GP or specialist mental health team. If you are able, share how you are feeling with family member or friend as well so they can support you to get help.

If you discussed with your doctor what to do if you get these thoughts and created a safety plan, read this to remind you who to contact for support and what you can do to help you cope.

Read our guide to suicidal feelings

Serious side effects

Mirtazapine can cause rare but serious side effects, including allergic reactions and bone marrow problems.

Stop taking mirtazapine and go to a hospital straight away if you get any of the following symptoms:

difficulty breathing
an allergic reaction or allergy (including symptoms such as swelling of your face, eyelids, lips, tongue or throat so that you cannot swallow or breathe, or really bad itching of the skin with raised lumps)
fast, uneven heartbeat and fainting (this could mean a life-threatening condition called Torsades de Pointes)
any combination of the following symptoms: high fever, agitation, confusion, trembling, sweating, diarrhoea (loose poo), fast heartbeat or weird movements of your muscles – this could be serotonin syndrome
you start having seizures or fits for the first time
a painful erection (priapism) that lasts more than two hours

Get your parent, family member or friend to take you or go with you to support you and keep you safe on the way. If someone is not able to drive you there, call for an ambulance. Take your medication with you.

Contact your doctor immediately but don't stop taking mirtazapine, if you get any of the following symptoms:

your behaviour changes because you feel very happy or over-excited
any unusual bleeding, including blood in your poo
you experience tiredness, confusion and muscle-twitching (you may have a low blood-level of sodium)
yellowing of the skin or whites of the eyes (these may be signs of hepatitis)
if fits that you have had in the past happen more often
restlessness, or difficulty sitting or standing still
dizziness, feeling sick or being sick
raised temperature (fever), sore throat, or mouth ulcers (These could be symptoms of problems with blood-cell production in the bone marrow, which can leave you more vulnerable to infections. While rare, these symptoms can appear after four to six weeks of treatment.)

If you are unable to contact your doctor, call NHS 111 for urgent advice.

Side effects and your health

Side effects are more common when you first start taking mirtazapine. Many go away as you continue your medicine. If they don’t get better, or if you are worried by them and they are causing you distress, speak to your doctor or pharmacist about them.

Very common side effects of mirtazapine, affecting more than one in ten people, include:

feeling hungrier and putting on weight
feeling drowsy or sleepy

Common side effects, affecting up to one in ten people, include:

not feeling like doing anything (lethargy) or feeling tired
feeling dizzy or shaking
feeling confused or anxious
feeling or being sick
diarrhoea (loose poo)
rash or skin problems
joint or muscle pain
feeling dizzy or faint when you stand up suddenly
swelling of the feet or ankles caused by fluid retention
sleep problems and unusual dreams

Young people aged under 18 are more likely than adults to put on weight, get an itchy rash, or get higher blood fat levels (triglycerides) in blood tests while taking mirtazapine.

There are other side effects that you can get when taking this medicine. We have only included the most common ones here.
This list of side effects can look scary. You may not experience any of them, but it’s important to be aware of them so you know what to do if they do happen.

Unless your side effects are very severe or distressing, try to continue taking your medication until you can talk to your doctor. Your doctor can make changes to your medication to help if needed.

Uncommon but important side effects to be aware of include thoughts of harming yourself, thinking about taking your own life or trying to take your own life. This may be a symptom of your illness, including depression, but may be a side effect of medication. If you experience these thoughts, contact your doctor to seek immediate support to keep you safe.

Your doctor will make sure you are monitored carefully for these side effects, and you should decide with your doctor what to do if you experience these feelings while taking amitriptyline. It’s a good idea to write these plans down and keep them somewhere safe.

If you are taking mirtazapine and have not talked about this with your doctor, go back to them and talk it through. You might also want to talk to your parents, family member or friend about it too.

Another uncommon side effect is increase in your risk of bleeding, including intestinal (stomach or gut) bleeding. Let your doctor know if you vomit blood or develop black or blood-stained poo.

There is no routine monitoring recommended before starting or while you are taking mirtazapine, although your doctor may recommend specific tests, like a blood test, to help them make decisions about your medication.

Because a very common side effect of this medicine includes increased appetite and weight gain, your doctor may recommend that your weight is monitored at regular intervals so that they can review your medication and provide support if needed.