Like all medicines, mirtazapine can cause side effects in some people, but many people have no side effects or only minor ones.
Some of the common side effects of mirtazapine will gradually improve as your body gets used to it.
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:
Dry mouth
Try chewing sugar-free gum or sucking sugar-free sweets.
Increased appetite and weight gain
Mirtazapine can make you hungrier so it can be quite a challenge to stop yourself putting on weight.
Try to eat well by having a healthy balanced diet without increasing your portion sizes. Do not snack on foods that contain a lot of calories, such as crisps, cakes, biscuits and sweets. If you feel hungry between meals, eat fruit and vegetables and low-calorie foods.
Regular exercise will also help to keep your weight stable.
Headaches
Make sure you rest and drink plenty of fluids. It's best not to drink too much alcohol. Ask your pharmacist to recommend a painkiller. Talk to your doctor if the headaches last longer than a week or are severe.
Feeling sleepy
Do not drive, cycle or use tools or machinery if you're feeling this way. It's best not to drink alcohol as this will make you feel more tired. If this becomes a problem your doctor may suggest that you switch to a different medicine.
Feeling or being sick (nausea or vomiting)
Stick to simple meals and do not eat rich or spicy food. It might help to take your mirtazapine after you've eaten. If you're being sick, try small frequent sips of water to avoid dehydration. Signs of dehydration include peeing less than usual or having dark, strong-smelling pee.
If you take the combined pill or progestogen-only pill and you're being sick your contraception may not protect you from pregnancy. Check the pill packet for advice.
Diarrhoea
Drink lots of fluids, such as water or squash, to avoid dehydration. Speak to a pharmacist if you have signs of dehydration such as peeing less than usual or having dark, strong-smelling pee. Do not take any other medicines to treat diarrhoea without speaking to a pharmacist or doctor.
If you take the combined pill or progestogen-only pill and you have severe diarrhoea for more than 24 hours, your contraception may not protect you from pregnancy. Check the pill packet for advice.
Constipation
Get more fibre into your diet such as fresh fruit and vegetables and cereals, and drink plenty of water. Try to exercise more regularly, for example, by going for a daily walk or run.
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
Serious side effects of mirtazapine are rare and happen in less than 1 in 10,000 people.
Call your doctor or call 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
Immediate action required: Call 999 or go to A&E now if:
- you have thoughts about harming yourself or ending your life
Do not drive to A&E. Ask someone to drive you or call 999 and ask for an ambulance.
Bring all the medicines you take with you.
Serious allergic reaction
In rare cases, it's possible to have a serious allergic reaction (anaphylaxis) to mirtazapine.
Immediate action required: Call 999 now if:
- your lips, mouth, throat or tongue suddenly become swollen
- you're breathing very fast or struggling to breathe (you may become very wheezy or feel like you're choking or gasping for air)
- your throat feels tight or you're struggling to swallow
- your skin, tongue or lips turn blue, grey or pale (if you have black or brown skin, this may be easier to see on the palms of your hands or soles of your feet)
- you suddenly become very confused, drowsy or dizzy
- someone faints and cannot be woken up
- a child is limp, floppy or not responding like they normally do (their head may fall to the side, backwards or forwards, or they may find it difficult to lift their head or focus on your face)
You or the person who's unwell may also have a rash that's swollen, raised, itchy, blistered or peeling.
These can be signs of a serious allergic reaction and may need immediate treatment in hospital.
Do not drive yourself to A&E.
The person you speak to at 999 will give you advice about what to do.
Other side effects
These are not all the side effects of mirtazapine. For a full list, see the leaflet inside your medicine packet.
Reporting side effects
You can report any suspected side effect using the Yellow Card safety scheme.
Visit the Yellow Card website for more information about reporting side effects.