HeadMeds gives young people in the United Kingdom general information about medication. HeadMeds does not give you medical advice. Please talk to your Doctor or anyone else who is supporting you about your own situation because everyone is different. Please read more important details about our site.

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Return to Mirtazapine overview
  1. Use and Action
  2. Warnings and side effects
  3. Sex, drink, weight and everything else
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Taking Mirtazapine and Amitriptyline: Sophie's story
I would've liked to have known it was used to treat a mental health condition and not "headaches" like I was originally told.

Please do not be worried by the side effects listed on this page. Many people take mirtazapine without any side effects or 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.

Mirtazapine can help to adjust the levels of the chemicals in your brain

For low mood / depression:

The brain has many naturally occurring chemical messengers (or "neurotransmitters"). Two of these are called serotonin (sometimes called 5-HT) and noradrenaline. Both 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. This can help correct the lack of action of these messengers and help to improve mood.

You should take mirtazapine as agreed with your doctor

    • You will get the best effect from mirtazapine if you take it regularly every day.
    • Make sure that you know your dose. If it is not written on the label, check it with your pharmacist or doctor.
    • Mirtazapine is taken once a day. It is best to take mirtazapine at bedtime because it can make you feel sleepy.
    • Although mirtazapine is taken at bedtime and can make you feel sleepy as a side effect, it is not a sleeping tablet
    • You can take your medicine before or after food.
    • For the plain tablets, swallow them whole with a drink of water or juice whilst you are sitting or standing up- if you chew them, they taste bitter.
    • This is to make sure that they reach the stomach and do not stick in your throat.
    • For the orodispersible (‘melt in your mouth’) tablets, peel off the cover from the blister pack rather than pushing the tablet through, so it does not break.
    • For the orodispersible (‘melt in your mouth’) tablets, put it on your tongue and let it dissolve there. 
    • There may be a measuring syringe to use with the oral solution to help you to 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 should I do if I forget to take a dose of mirtazapine?

  • 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.
  • Mirtazapine works best if you do not miss any doses. However, as long as 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.
  • Leaving the pack of mirtazapine somewhere you will see it each evening may help you to remember to take it (such as in the bathroom or by the bed).

You must go to A&E if you take too much 

What to do if you take too much:

  • If you have taken more mirtazapine than it said on the label, you must get help quickly – 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

It can take a week or two for mirtazapine to start helping

  • 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.
  • Some side effects of mirtazapine tend to happen at the start of treatment, but go away after a few days, such as feeling or being sick (nausea or vomiting).
  • To get the best effect, you need to take your mirtazapine every night and give it a chance to work for you.
  • Your doctor might start you on a low dose and then increase it slowly over a few weeks to your full dose.

Many people take mirtazapine for longer than 6 months

  • Keep taking mirtazapine as you get better, which can take a few months, and then keep taking the mirtazapine for another 6 to 12 months 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 mental health symptoms will come back.
  • Discuss with your doctor how long you should take mirtazapine for.
  • See your doctor if you want to stop mirtazapine, to make sure it is not too soon to stop it.
  • Also, the mirtazapine dose will need to be reduced gradually to reduce the chance of side effects when it is stopped (unless you are already on the lowest dose).

Tell your doctor or pharmacist before you take mirtazapine if you have any of these conditions

You need to talk to your doctor or pharmacist before you take mirtazapine if you have ever had:

  • fits (seizures)
  • liver disease, including jaundice
  • kidney disease
  • heart disease, or low blood pressure
  • schizophrenia
  • manic depression / bipolar disorder
  • diabetes
  • eye disease, such as increased pressure in the eye (glaucoma);
  • difficulty in passing water (urinating), which might be caused by an enlarged prostate.
  • If you have ever tried to harm or kill yourself, or thought about harming or killing yourself, you should tell your doctor. Early on in treatment these thoughts might come back.

If you have any thoughts of suicide, or of other ways of hurting yourself, go straight to a hospital with your tablets.

This may be a side-effect, and you need urgent help.

  • Whilst taking mirtazapine some people may think, some people think about hurting themselves orhave suicidal thoughts. This is more common at the start of treatment than later on.
  • This can happen to anyone, but is more likely to happen if you are less than 25 years old.
  • You must to straight to hospital if you have thoughts of hurting yourself or have suicidal thoughts. Take your medicines with you and tell the doctor that you are taking mirtazapine

Mirtazapine has possible side effects, and some of these are rare but serious

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

  • Feeling like killing 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 4-6 weeks of treatment.
  • Yellow colouring of the skin and the white 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 blood level of sodium, but some of these are also symptoms of serotonin syndrome.

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

Some side-effects of mirtazapine may – strangely - seem like other mental health symptoms. Some side-effects here are also the opposites of each other. The balance of chemicals in the brain is very fragile, and hard to control! If they do not get better after a few days on the tablets, go back to the doctor.

Young people were more likely than adults to get some side-effects in tests

  • Young people aged under 18 were 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 aged under 25 were more likely to have thoughts about harming themselves or taking their own lives than older adults.
  • 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.
  • This is why it is not licensed for people under 18 years.
  • If you are under 18, make sure that you and your doctor have talked about what to do if you get these side effects.
  • You and your doctor may decide together that the benefits are greater than the risks for you.
  •  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 or carers about it.

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.

If you have any further questions about this you should speak to your doctor or pharmacist.

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 medicines (including things you put on your skin) for common illnesses.

Mirtazapine does not mix well with street drugs 

  • Cannabis can make drowsiness worse with mirtazapine and give you a fast heartbeat.
  • 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. With amphetamines there is likely to be increased antidepressant response to this stimulant but this combination should only be used close medical supervision.

Stopping this medicine quickly, or reducing the dose too much at once, may cause your old symptoms to come back, or withdrawal symptoms

 You can stop taking it safely with your doctor’s help

  • Once you start taking mirtazapine, the brain adjusts to having a new level of noradrenaline and serotonin around.
  • You will probably get uncomfortable withdrawal symptoms if you stop mirtazapine suddenly. These symptoms include dizziness, agitation, anxiety, headache, feeling or being sick
  • They are usually mild and stop after a few days
  • This is not because mirtazapine is addictive – it is all about the brain re-adjusting to a change in balance of the transmitters.
  • It is better to agree stopping with a doctor who will reduce your dose gradually over a few weeks

Reference Sources 

Search www.medicines.org.uk to find patient information leaflets and prescribing information on mirtazapine. Tablets, orodispersible tablets and liquid are listed separately. The SmPC lists all the inactive ingredients in the product so you can check against any allergies. If you are still unsure about this then speak to your pharmacist.

  • British National Formulary (BNF) and British National Formulary for children. Download the BNF/BNFC app (blue background) on to your mobile device. No longer available for public access via the web 
  • Taylor D, Barnes T, Young A. Maudsley Prescribing Guidelines in Psychiatry, 13th edition. Oxford: John Wiley & Sons, May 2018. ISBN: 978-1-119-44260-8
  • Royal Pharmaceutical Society. Medicines Ethics and Practice (42nd edition). London: RPS, 2018. Standards for pharmacists to work to. It is not a free publication
  • World Anti Doping Agency WADA Prohibited List https://www.wada-ama.org/en/resources/science-medicine/prohibited-list-documents 
  • Choiceandmedication; an independent source of information on many mental health conditions and their medicines with easy to read fact sheets www.choiceandmedication.org Personal subscriptions to download the app available for £1 per month (with proportionate discounts for longer periods) but your local mental health Trust may subscribe and provide information sheets for free.
  • Best Use of Medicines in Pregnancy (BUMPS). Information sheets on drugs in pregnancy http://www.medicinesinpregnancy.org/ 
  • Drugs and Lactation Database (LactMed). Information on drugs in breastfeeding https://toxnet.nlm.nih.gov/newtoxnet/lactmed.htm

Some side-effects that do appear should get better after a few days. If they do not, you should go back to your doctor.

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

Very common - could affect more than 1 in 10 people

  • feeling more hungry and putting on weight
  • feeling drowsy or sleepy
  • Common - could affect up to 1 in 10 people

    • not feeling like doing anything (lethargy), or feeling tired
    • feeling dizzy or shaking
    • feeling confused or anxious
    • feeling or being sick
    • loose poo (diarrhoea)
    • rash or skin problems (exanthema)
    • pain in your joints (arthralgia) or muscles (myalgia) or back
    • feeling dizzy or faint when you stand up suddenly (orthostatic hypotension)
    • swelling – usually in your ankles or feet - caused by fluid retention (oedema)
    • sleeping problems and unusual dreams