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.
Keep taking the medicine, but tell your GP, a pharmacist or nurse if side effects bother you or do not go away.
Common side effects
The common side effects of mirtazapine happen in more than 1 in 100 people. That means they do not happen in 99 in 100 people.
There are things you can do to help you cope with common side effects:
Diarrhoea
Drink lots of fluids, such as water, 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 medicines to treat diarrhoea without speaking to a pharmacist or doctor.
If you take contraceptive pills and you have severe diarrhoea for more than 24 hours, your contraception may not protect you from pregnancy. Check the pill packet for advice.
Feeling sick (nausea)
Stick to simple meals and do not eat rich or spicy food. It might help to take your mirtazapine after you've eaten.
Getting sick (vomiting)
If you're getting 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 contraceptive pills and you're getting sick, your contraception may not protect you from pregnancy. Check the pill packet for advice.
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 hard not to put 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 GP 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 GP may suggest that you switch to a different medicine.
Constipation
Speak to a GP 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.
See the patient information leaflet that comes with your medicine for a full list of side effects.
Non-urgent advice: Find your patient information leaflet
Your patient information leaflet is the leaflet that comes with your medicine. You can find a digital version of the leaflet online.
Report side effects
You can report any suspected side effects to the Health Products Regulatory Authority (HPRA): report an issue - hpra.ie
Thoughts of harming yourself
Thoughts of harming yourself may increase if you have depression and start taking mirtazapine.
Non-urgent advice: Phone your GP or go to an emergency department (ED) straightaway if:
- you you have thoughts of harming yourself while taking mirtazapine
You may be more likely to think like this if:
- you have previously had thoughts about harming yourself
- you are around age 25 or younger
You may find it helpful to tell a relative or close friend that you are depressed. Ask them to tell you if they think your depression is getting worse, or if they are worried about changes in your behaviour.
Serious side effects
Urgent advice: Call your GP straight away and stop taking mirtazapine if you have:
- severe pain in your stomach or back, and nausea
- constant headaches, long-lasting confusion or weakness, or frequent muscle cramps
- yellow skin, or the whites of your eyes go yellow
- high fever, sore throat and mouth ulcers
- feelings of elation or are emotionally ‘high’ (mania)
- seizures (fits)
- a mix of symptoms, such as fever, sweating, faster heart rate, diarrhoea, muscle spasms, shivering, restlessness, mood changes, unconsciousness and increased spit
- red circle patches on your chest, often with blisters, skin peeling, or mouth, throat, nose, genitals or eye ulcers
- widespread rash and high temperature of 38 degrees Celsius or higher
Serious allergic reaction
A serious allergic reaction to mirtazapine is rare.
Emergency action required: Call 112 or 999 or go to an emergency department (ED) straight away if:
- you get a skin rash that is itchy, red, swollen, blistered or peeling
- you're wheezing
- you get tightness in the chest or throat
- you have trouble breathing or talking
- your mouth, face, lips, tongue or throat start swelling
These are warning signs of a serious allergic reaction. You might need to go to hospital.