Mirtazapine is an antidepressant medicine. It's used to treat depression and sometimes obsessive compulsive disorder and anxiety disorders.

Mirtazapine is available only on prescription. It comes as tablets or as a liquid you swallow.

• Mirtazapine usually takes about 4 to 6 weeks to work.
• Common side effects include headaches, dry mouth and feeling sick. They're usually mild and go away after a couple of weeks.
• If you and your doctor decide to take you off mirtazapine, your doctor will probably recommend reducing your dose gradually to help prevent extra side effects.
• Mirtazapine is not a sleeping tablet but it can make you feel sleepy. This can be helpful if you have depression and difficulties getting to sleep.
• Mirtazapine is also known by the brand name Zispin SolTab.

Mirtazapine can be taken by adults for depression, obsessive compulsive disorder and anxiety disorders.

Mirtazapine isn't suitable for some people. Check with your doctor before starting to take mirtazapine if you:

• have had an allergic reaction to mirtazapine or any other medicines in the past
• 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 if you've stopped taking them for a few weeks
• have an eye problem called 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 become pregnant, already pregnant or breastfeeding
• are taking warfarin - mirtazapine may increase the effects of warfarin and blood clotting

If you have diabetes, mirtazapine can make it more difficult to keep your blood sugar stable. Monitor your blood sugar more often for the first few weeks of taking mirtazapine and change your diabetes treatment if necessary.

You'll usually take mirtazapine once a day. It's best to take mirtazapine before you go to bed as it can make you sleepy.

Your doctor may recommend dividing your daily dose into 2 doses of different sizes. In this case take the smaller dose in the morning and the higher dose before you go to bed.

Mirtazapine can be taken with or without food.

How much will I take?

The usual starting dose for mirtazapine is 15 to 30mg a day. This can be increased up to 45mg a day.

If you have problems with your liver or kidneys your doctor might prescribe a lower dose.

What if I forget to take it?

If you take mirtazapine once a day and miss a dose, skip it and take the next dose at the normal 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 normal morning and evening doses

• both doses – skip the missed doses. Continue the next day with your normal morning and evening doses. Do not take an extra dose to make up for a missed dose

If you forget doses often, 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.

What if I take too much?

The amount of mirtazapine that can lead to an overdose varies from person to person.

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 happen in more than 1 in 100 people.

Keep taking the medicine, but tell your doctor or pharmacist if these side effects bother you or don't go away:

• dry mouth
• increased appetite and weight gain
• headaches
• feeling sleepy
• constipation

Serious side effects

Serious side effects are rare and happen in less than 1 in 10,000 people when taking mirtazapine.

Call your doctor straight away if you experience:

• severe pain in your stomach or back, and nausea – these can be signs of inflammation of the pancreas (pancreatitis)
• thoughts about harming yourself or ending your life
• constant headaches, long-lasting confusion or weakness, or frequent muscle cramps – these can be signs of low sodium levels in your blood (which can cause seizures in severe cases)
• yellow skin, or the whites of your eyes go yellow – this can be a sign of liver problems
• high fever, sore throat and mouth ulcers – these signs of infection could be due to a problem with your blood cells

Serious allergic reaction

In rare cases, it's possible to have a serious allergic (anaphylaxis) reaction to mirtazapine.

These are not all the side effects of mirtazapine. For a full list, see the leaflet inside your medicine packet.

What to do about:

• 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 a healthy balanced diet without increasing your portion sizes. Don't 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. Don't drink too much alcohol. Ask your pharmacist to recommend a painkiller. Talk to your doctor if headaches last longer than a week or are severe.
• feeling sleepy – do not drive or use tools or machinery if you're feeling this way. Try to avoid drinking alcohol as this will make you feel more tired. Talk to your doctor if this becomes troublesome as you may need to switch to a different medicine.
• constipation – eat more high-fibre foods 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. If this doesn't help, talk to your pharmacist or doctor.

It's important for you and your baby that you stay well during your pregnancy.

If you become pregnant while taking mirtazapine speak to your doctor. Do not stop taking your medicine unless your doctor tells you to.

Mirtazapine has been linked to a very small increased risk of problems for your unborn baby.

But if your depression is not treated during pregnancy this can also increase the chance of problems.

You may take mirtazapine during pregnancy if you need it to remain well. Your doctor can explain the risks and the benefits, and will help you decide which treatment is best for you and your baby.

For more information about how mirtazapine can affect you and your baby during pregnancy, read the leaflet about the best use of medicines in pregnancy (BUMPS).

Mirtazapine and breastfeeding

If your doctor or health visitor says your baby is healthy, mirtazapine can be used during breastfeeding. It has been used by many breastfeeding mothers usually without any problems.

Mirtazapine passes into breast milk in small amounts. It has been linked with side effects in very few breastfed babies.

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 isn't feeding as well as usual, or seems unusually sleepy, or if you have any other concerns about your baby, talk to your health visitor or doctor as soon as possible.

Some medicines and mirtazapine can interfere with each other and increase the chances of you having side effects.

Tell your doctor if you are taking:

• or recently been on any other medicines for depression – some antidepressants can interfere with 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 interfere with warfarin so you may need to change your warfarin dose
• antiepileptics such as carbamazepine and 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

Mixing mirtazapine with herbal remedies and supplements

Do not take St John's wort while you are being treated with mirtazapine as this will increase your risk of side effects.