It works by increasing the amount of mood-enhancing chemicals called noradrenaline and serotonin in your brain.
Mirtazapine is only available on prescription. It comes as tablets, tablets that dissolve in your mouth or as a liquid that you swallow.
Who can take mirtazapine
Most adults can take mirtazapine.
If you have diabetes, mirtazapine can make it more difficult to keep your blood sugar stable. Your doctor may recommend that you monitor your blood sugar more often for the first few weeks of taking it and adjust your diabetes treatment if necessary.
Mirtazapine is not suitable for some people. To make sure it's safe for you, tell your doctor before starting to take mirtazapine if you:
- have ever had an allergic reaction to mirtazapine or any other medicine
- have a heart problem as mirtazapine can cause low blood pressure
- have ever taken any other medicines for depression – some rarely used antidepressants can interfere with mirtazapine to cause very high blood pressure even a few weeks after you've stopped taking them
- have glaucoma – mirtazapine can increase the pressure in your eye
- have epilepsy – although it's rare, mirtazapine may increase your risk of having a seizure
- are trying to get pregnant, already pregnant or breastfeeding
The usual starting dose for mirtazapine is 15mg to 30mg a day. This can be increased to up to 45mg a day.
Your doctor may recommend dividing your daily dose into 2 doses of different sizes.
If you have problems with your liver or kidneys your doctor might prescribe a lower dose.
You can take mirtazapine with or without food.
Swallow the tablets whole with a drink of water. Do not break, crush or chew them.
Mirtazapine also comes as tablets that melt in your mouth (orodispersible tablets). Put the tablet on your tongue and let it dissolve. You can then swallow it without a drink.
Liquid mirtazapine will come with a plastic syringe or spoon to help you measure out the right dose. If you do not have a syringe or spoon, ask your pharmacist for one. Do not use a kitchen teaspoon as it will not measure the right amount.
You'll usually take mirtazapine once a day. It's best to take it before you go to bed as it can make you sleepy.
If your doctor tells you to take it twice a day, take the smaller dose in the morning and the higher dose before you go to bed.
How long to take it for
Once you're feeling better you're likely to keep taking mirtazapine for several more months.
Most doctors recommend that you take antidepressants for 6 months to a year after you no longer feel depressed.
Stopping your medicine too soon can make depression come back.
If you forget to take it
If you take mirtazapine once a day and miss a dose, skip the missed dose and take the next dose at the usual time. Do not take a double dose to make up for a forgotten dose.
If you take mirtazapine twice a day and forget:
- your morning dose – take it together with your evening dose
- your evening dose – do not take it with the next morning dose. Instead skip the missed dose, and then continue the next day with your usual morning and evening doses
- both doses – skip the missed doses. Continue the next day with your usual morning and evening doses. Do not take an extra dose to make up for a missed dose
If you often forget doses, it may help to set an alarm to remind you. You could also ask your pharmacist for advice on other ways to help you remember to take your medicine.
If you take too much
Taking more than your prescribed dose of mirtazapine can cause problems. You may get symptoms including:
- feeling sleepy
- a fast or irregular heartbeat
- feeling confused or faint
The amount of mirtazapine that can lead to an overdose varies from person to person.
- you've taken more than your prescribed dose of mirtazapine
If you need to go to A&E, do not drive yourself. Get someone else to drive you or call for an ambulance.
Take the mirtazapine packet, or the leaflet inside it, plus any remaining medicine with you.
If you've been feeling better for 6 months or more, your doctor may suggest coming off mirtazapine.
Your doctor may recommend reducing your dose gradually over several weeks, or longer if you've been taking mirtazapine for a long time.
This is to help prevent you getting any extra side effects as you come off the medicine.
Do not stop taking mirtazapine suddenly, or without talking to your doctor first.
Common side effects
These side effects of mirtazapine happen in more than 1 in 100 people. There are things you can do to help cope with them:
Speak to a doctor or pharmacist if the advice on how to cope does not help and a side effect is still bothering you or does not go away.
Serious side effects of mirtazapine are rare and happen in less than 1 in 10,000 people.
Call your doctor or contact 111 straight away if:
- you get severe pain in your stomach or back, and nausea – these can be signs of inflammation of the pancreas (pancreatitis)
- you have constant headaches, long-lasting confusion or weakness, or frequent muscle cramps – these can be signs of low sodium levels in your blood
- the whites of your eyes turn yellow, or your skin turns yellow although this may be less obvious on brown or black skin – this can be a sign of liver problems
- you get a high temperature, sore throat and mouth ulcers – these are signs of infection that could be due to a problem with your blood cells
- you have thoughts about harming yourself or ending your life
In rare cases, it's possible to have a serious allergic reaction (anaphylaxis) to mirtazapine.
These are not all the side effects of mirtazapine. For a full list, see the leaflet inside your medicine packet.
Mirtazapine and pregnancy
There's no good evidence that taking mirtazapine in early pregnancy will affect your baby's development.
When you take mirtazapine in the weeks before delivery it might sometimes cause short-term withdrawal symptoms and, very rarely, breathing problems in the baby. Your baby will be checked after birth and given extra care if needed.
Taking mirtazapine in the last month of pregnancy may slightly increase your risk of bleeding after delivery. However, as this side effect is rare, most women choose to continue mirtazapine while pregnant.
It is important that mental health problems are well treated since these can affect both you and your baby's wellbeing. Depression and anxiety can sometimes get worse during pregnancy, and after the baby's born.
Speak to your doctor if you become pregnant. They will help you weigh up the risks and benefits so you can decide on the best treatment for you and your baby.
If your doctor or health visitor says your baby is healthy, you can take mirtazapine while breastfeeding.
Mirtazapine passes into breast milk in small amounts and has often been used during breastfeeding without any problems.
Although other medicines might be preferred while you are breastfeeding, it is important you take the medicine that works for you. If you are breastfeeding, or planning to breastfeed, talk to your doctor or pharmacist to help you decide what is best for you.
It's important to continue taking mirtazapine to keep you well. Breastfeeding will also benefit both you and your baby.
If you notice that your baby is not feeding as well as usual, seems unusually sleepy, or irritable and restless, or if you have any other concerns about your baby, talk to your health visitor, midwife, pharmacist or doctor as soon as possible.
Mirtazapine and fertility
There's no clear evidence to suggest that mirtazapine affects fertility in either men or women.
Speak to your doctor if you're planning to get pregnant while taking mirtazapine.
Cautions with other medicines
Some medicines and mirtazapine can affect each other and increase the chances of you having side effects.
Tell your doctor or pharmacist if you're taking:
- any other medicines for depression, or if you've taken them recently – some antidepressants can affect mirtazapine to cause very high blood pressure even after you've stopped taking them
- medicines that make you feel sleepy, including strong painkillers like morphine or muscle relaxants like diazepam
- warfarin – mirtazapine can affect warfarin so your doctor may need to change your warfarin dose
- epilepsy medicines such as carbamazepine or phenytoin – these medicines can reduce the effects of mirtazapine, so your doctor may prescribe a higher dose
- rifampicin – this antibiotic can reduce the effects of mirtazapine, so your doctor may prescribe a higher dose
Do not take the herbal remedy St John's wort while you're being treated with mirtazapine, as this will increase your risk of side effects.
There's not enough information to say that other herbal remedies or supplements are safe to take with mirtazapine. They're not tested in the same way as pharmacy and prescription medicines.
HealthUnlocked contains information from NHS Digital, licensed under the current version of the Open Government Licence