Mirtazapine

How it works

What does mirtazapine do?

Mirtazapine is an antidepressant, which works by adjusting the levels of chemicals in your brain.

The brain has many naturally occurring chemical messengers (or "neurotransmitters"). Two of these - serotonin (sometimes called 5-HT) and noradrenaline - are important in the areas of the brain that control mood and thinking.

It is known that these chemical messengers are not as effective or active as usual in the brain when someone is feeling depressed. Mirtazapine increases the amount of these chemical messengers in the brain, which can help to improve mood.

How long does mirtazapine take to start working?

Antidepressants like mirtazapine can start to work on depression within the first two weeks of treatment, and the improvement continues over the following few weeks. It may take four weeks or a little longer for you to get 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 long will I need to take mirtazapine?

Keep taking mirtazapine as you get better, which can take a few months, and then continue taking it for another six to 12 months after that as advised by your doctor.

If your illness has come back, then you should keep taking mirtazapine for at least two years after getting better. This will help keep you well.

If you stop taking the mirtazapine too soon, there is more chance that your symptoms will come back.

Discuss with your doctor how long you should take mirtazapine before starting your treatment.

If you want to stop mirtazapine, speak to your doctor to make sure it is not too soon.

Your doctor needs to know if...

You need to talk to your doctor or pharmacist before starting treatment with mirtazapine if you have experienced any of the following:

• seizures (fits)
• liver disease, including jaundice
• kidney disease
• heart disease, or low blood pressure
• schizophrenia
• bipolar disorder
• diabetes
• eye disease, such as glaucoma (increased pressure in the eye)
• difficulty urinating (weeing), which might be caused by an enlarged prostate

If you have ever tried to harm yourself or take your own life, or thought about harming yourself or taking your own life, you should tell your doctor, as these thoughts might come back early on in treatment.

The mirtazapine tablets that you swallow may not be suitable for you if you have problems eating some sugars or dairy (milk-based) 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 a small amount of alcohol. It also contains maltitol, which can be a problem for anyone who has an intolerance to a sugar called fructose.

Mirtazapine medicines do not contain any animal products.

Taking mirtazapine

You should only take mirtazapine as agreed with your doctor

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

Make sure that you know your dose. If it is not written on the label, check with your pharmacist or doctor.

Mirtazapine is 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 (melts), 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.

There may be a measuring syringe to use with the oral solution to help you get the dose right. Read the instructions before you use it.

You can add your dose of the oral solution to a glass of water.

What if I miss a dose?

If you forget your bedtime dose of mirtazapine and you remember in the morning, then it is probably best that you miss that dose. This is because mirtazapine can make you sleepy, so it is best not to take it in the daytime. Just take your next dose at bedtime as usual.

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

What will happen if I forget to take my mirtazapine?

Mirtazapine works best if you do not miss any doses. However, if you are not missing more than one dose each week, it is unlikely that there will be any problems.

If you forget to take your tablets for a few days, you may start getting your old symptoms back, or you may get withdrawal symptoms such as headache, dizziness, anxiety, or feeling sick. You should talk to your doctor about this.

Stopping the use of mirtazapine

Stopping this medicine quickly, or reducing the dose too much at once, may cause your old symptoms to come back. It may also give you unpleasant withdrawal symptoms including dizziness, agitation, anxiety, headaches and feeling or being sick. These are usually mild and go away after a few days.

Once you start taking mirtazapine, the brain adjusts to having a new level of noradrenaline and serotonin around. Withdrawal symptoms are not caused by the medication being addictive, it’s just your brain readjusting to a change in the balance of chemicals.

It is better to agree stopping with a doctor who will reduce your dose gradually over a few weeks.

The information on this page was reviewed by the College of Mental Health Pharmacy in March 2020.

Safety headlines

If you have taken more mirtazapine than the dosage recommended by the doctor who prescribed it to you, you must get medical help immediately – even if you do not feel any different.

Mirtazapine can make some people think about hurting themselves or taking their own lives. You must go straight to hospital with your tablets if you have any of these thoughts.

Mirtazapine can also cause rare but serious side effects: allergic reactions (difficulty breathing, swelling of your face or throat, itching skin lumps), and bone marrow problems (symptoms could be high fever, sore throat or mouth ulcers). Go to a hospital with your medicine if you get any of these symptoms.

Do not take mirtazapine if you have taken a monoamine oxidase inhibitor antidepressant (MAOI) like moclobemide, phenelzine, isocarboxazid or tranylcypromine in the last 14 days.

Stopping mirtazapine suddenly can cause unpleasant withdrawal effects. Go to your doctor if you want to stop, or if you are having these effects.

You might feel sleepy or dizzy, or less alert, in the first few days after taking mirtazapine. Do not drive a car, ride a bike or operate machines until you see how this affects you.

If you are pregnant, or thinking of becoming pregnant, please read the pregnancy, post-natal and breastfeeding section under the 'Side Effects' tab, as mirtazapine may affect the developing baby.

When to go to the hospital

If you have taken more mirtazapine than the dosage recommended by the doctor who prescribed it to you, you must get medical help immediately – even if you do not feel any different. Go to A&E. Take your medicine with you to show to the doctors. Tell them how much you have taken. Get a friend or family member to go with you, if you can, just in case you feel ill on the way.

You might get any of the following signs:

• feeling sleepy or disorientated
• having a fast heartbeat

If you have any thoughts of taking your own life or of other ways of hurting yourself, go straight to a hospital with your tablets. This may be a side effect, but you need urgent help. If you’re less than 25 years old, you’re more likely to be affected by this and it’s also more likely to happen at the beginning of your treatment.

Stop taking your mirtazapine and go to a doctor or hospital straight away if you get the following symptoms:

• feeling like taking your own life or hurting yourself (see above)
• 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
• yellowing of the skin and the whites of the eyes. This is called jaundice, and it can be a symptom of liver problems
• rashes, blotches, itching, blistering, redness, peeling, or ulcers on your skin, in your mouth, or in your genital area. These can be symptoms of a rare but serious skin reaction
• having fits (also known as convulsions or seizures)
• feeling very excited or ‘high'
• shivering, excessive sweating, restlessness, irritability, agitation, confusion, mood changes, high temperature (fever), fast heartbeat, diarrhoea, trembling, weird muscle movements (that you cannot control), overactive reflexes, or losing consciousness. These may be symptoms of a rare but serious condition called serotonin syndrome
• tiredness, confusion, headache, irritability, feeling or being sick (nausea and vomiting), and muscle twitching. These can be symptoms of a low level of sodium in your blood, but some of these are also symptoms of serotonin syndrome

Side effects

Please do not be worried by the side effects listed on this page. Many people take mirtazapine without any side effects or with only a few mild side effects. If you think you might be getting a side effect from mirtazapine, then you should discuss this with your doctor, nurse, or pharmacist.

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

Young people under 18 have an increased risk of trying to take their own lives, thinking about taking their own lives, and hostility (mostly aggression, oppositional behaviour and anger) when they take mirtazapine. Therefore, it is not licensed for people under 18 years.

Your doctor may decide that mirtazapine is the best medication for you to take, even if you are under 18. They will discuss the reasons for this decision with you and will talk to you about what to do if you experience side effects.

If you get any side effects not listed here please look at the patient leaflet in the medicine pack.

Don't stop taking mirtazapine until you talk to your doctor or you may get withdrawal symptoms as well.

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

Body

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

Talk to your doctor about this if it worries you.

Sleep

Mirtazapine can make you feel very sleepy.

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

Talk to your doctor about this if it happens to you and does not get better after a few days.

Sex and fertility

Sex

Mirtazapine can have side effects that might affect your sex life, though these aren’t usually significant.

The good effects of mirtazapine may have a positive impact on your sex life as your symptoms settle and you can concentrate on your relationships. 

Fertility

There is nothing to suggest that mirtazapine affects fertility.

Pregnancy, post-natal and breastfeeding

Pregnancy

Studies of over 600 mums taking mirtazapine in the first trimester show no increase in problems in the baby, nor risk of malformations when usual doses have been taken.

There may be a very slight increase in the risk of miscarriage or having a baby born early.

If mirtazapine is taken after 20 weeks of pregnancy, there is in theory a risk of Persistent Pulmonary Hypertension in the New-born (PPHN), but there is not enough research to say if this happens with mirtazapine or not. PPHN may cause breathing problems in your baby.

Talk to your doctor if you want to start trying for a baby while taking mirtazapine or if you become pregnant while on this medication.

Post-natal

Your baby could have some discontinuation symptoms such as being irritable, crying, shivering or problems sleeping. These are usually mild and go away in a few days without treatment.

Make sure that your doctor, nurse, or health visitor checks your baby for any side effects. These can include:

• being extra sleepy
• having colic
• feeding problems
• being floppy
• poor weight gain

Your baby’s blood sugar is likely to be checked as in rare cases it may be too low.

Breastfeeding

Mirtazapine is passed to the baby in breast milk in small amounts, but this can help with any discontinuation symptoms.

Remember that it is important for you to remain well while you are bonding with and looking after your baby. For this reason, it may be best to take medicine for your mental health when breastfeeding.

You may also wish to consider bottle-feeding with formula milk if there are any problems with breastfeeding while taking medicines.

Talk to your doctor or midwife about your feeding options.

Driving and transport

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, until you know how it affects you or the side effects pass.

Don't worry - most people drive as normal while taking mirtazapine.

School and exams

Try not to take mirtazapine for the first time just before your exams.

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

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 to study.

Don't worry - most people take exams as normal while taking mirtazapine.

Friends and family

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.

Sport

Mirtazapine is not a banned substance in sport.

You might, however, feel sleepy or less alert when taking it.

If you play sport that needs a lot of focus, it might be best to stop until you know how mirtazapine affects you.

Don't worry - most people play sports as normal while taking mirtazapine.

Alcohol, street drugs and smoking

Alcohol

You can continue to drink alcohol while taking mirtazapine, but having the two together might make you very sleepy and unsteady on your feet.

During the first few days, it might be best to stop drinking alcohol until you see how the medicine affects you, or the side effects pass.

If you want to drink alcohol, remember that you might be very sleepy and make sure you can get home safely.

Drinking alcohol every day, or in large amounts, can make your symptoms worse and the mirtazapine will not get the best chance to act.

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. For more information, have a look at our drugs and alcohol page.

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 is likely to dampen the stimulant effects of cocaine and the same with ecstasy. 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.

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 (re)start smoking, you will probably need to increase your dose of mirtazapine again.

Go to your doctor for advice if you stop or start smoking

Prescription medicines

Mirtazapine does not mix well with some other medicines and drugs.

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.

Tell your doctor or pharmacist before taking mirtazapine if you are taking any other medicines.

Tell the pharmacist you are taking mirtazapine if you buy over-the-counter medicines (including things you put on your skin) for common illnesses.

References and further reading

For more helpful links and information, have a look at our references and further reading page.