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.