Peginterferon alfa 2a

Peginterferon alfa 2a is an immunotherapy Open a glossary item drug. It is also known as Pegasys. You pronounce peginterferon alfa 2a as peg-in-ter-feer-on-al-far-2-a.

It is a possible treatment for myeloproliferative disorders such as polycythaemia vera (PV) and essential thrombocythaemia (ET).

It is also used to treat conditions other than cancer including hepatitis B and hepatitis C.

How does peginterferon alfa 2a work?

Peginterferon alfa 2a stimulates cells of the immune system Open a glossary item to attack the cancer. It can also encourage cancer cells to send out chemicals that attract immune system cells to them.

How do you have peginterferon alfa 2a?

You usually have peginterferon alfa 2a as an injection just under the skin (subcutaneously). Peginterferon alfa 2a comes as a pre filled syringe.

A nurse may teach you to give the injection yourself at home if you are having this drug for some weeks or months. Or a family member can learn how to do it.

You need to keep your peginterferon alfa 2a in the fridge.

You usually have injections under the skin (subcutaneous injection) into the stomach, thigh or top of your arm.

You might have stinging or a dull ache for a short time after this type of injection but they don't usually hurt much. The skin in the area may go red and itchy for a while.

The video below shows you how to give an injection just under your skin (subcutaneously).

How often do you have peginterferon alfa 2a?

You have peginterferon alfa 2a once a week. This might be reduced to every 2 or 3 weeks if your blood test results are good. You continue to have peginterferon alfa 2a as long as it is helping and the side effects aren’t too bad.


You have blood tests before and during your treatment. They check your levels of blood cells and other substances in the blood. They also check how well your liver and kidneys are working.

What are the side effects of peginterferon alfa 2a?

Side effects can vary from person to person. They also depend on what other treatments you're having. 

When to contact your team

Your doctor, nurse or pharmacist will go through the possible side effects. They will monitor you during treatment and check how you are at your appointments. Contact your advice line as soon as possible if:

  • you have severe side effects 

  • your side effects aren’t getting any better

  • your side effects are getting worse

Early treatment can help manage side effects better. 

Contact your advice line immediately if you have signs of infection, including a temperature above 37.5C or below 36C.

We haven't listed all the side effects here. Remember it is very unlikely that you will have all of these side effects. But you might have some of them at the same time.

Common side effects

These side effects happen in more than 10 in 100 people (more than 10%). You might have one or more of them. They include:

Difficulty sleeping

It can help to change a few things about how you try to sleep. Try to go to bed and get up at the same time each day and spend some time relaxing before you go to bed. Some light exercise each day may also help.

Mood changes

You might have a change in your mood. Changes can include irritability, excitement, restlessness or depression.


Tell your healthcare team if you keep getting headaches. They can give you painkillers to help.


This drug might make you feel dizzy. You might also feel dizzy when standing or moving around. Or it might feel like the world is spinning.

Don’t drive or operate machinery if you have this.

Memory problems and difficulty concentrating

Let your nurse or doctor know if you have this. 

Feeling or being sick

Feeling or being sick is usually well controlled with anti sickness medicines. It might help to avoid fatty or fried foods, eat small meals and snacks and take regular sips of water. Relaxation techniques might also help.

It is important to take anti sickness medicines as prescribed even if you don’t feel sick. It is easier to prevent sickness rather than treat it once it has started.

Tummy (abdominal) pain

Tell your treatment team if you have this. They can check the cause and give you medicine to help.


Contact your advice line if you have diarrhoea. For example, in one day you have 2 or more loose bowel movements than usual. If you have a stoma, you might have more output than normal. Your doctor may give you anti diarrhoea medicine to take home with you after treatment.

Try to eat small meals and snacks regularly. It’s best to try to have a healthy balanced diet if you can. You don’t necessarily need to stop eating foods that contain fibre. But if your diet is normally very high in fibre, it might help to cut back on high fibre foods such as beans, nuts, seeds, dried fruit, bran and raw vegetables. 

Drink plenty to try and replace the fluid lost. Aim for 8 to 10 glasses per day.

Hair loss

You could lose all your hair. This includes your eyelashes, eyebrows, underarms, legs and sometimes pubic hair. Your hair will usually grow back once treatment has finished but it is likely to be softer. It may grow back a different colour or be curlier than before. 

Pain in your muscles and joints

You might feel some pain in your muscles and joints. Speak to your doctor or nurse about what painkillers you can take to help with this.

Inflammation at the injection site

You might notice redness, swelling and pain around the area where you have had the injection.

Tiredness (fatigue)

You might feel very tired and as though you lack energy.

Various things can help you to reduce tiredness and cope with it, for example exercise. Some research has shown that taking gentle exercise can give you more energy. It is important to balance exercise with resting.

Flu-like symptoms

This can happen a few hours after treatment. It may include headaches, muscle aches (myalgia), a high temperature and shivering. Taking paracetamol every 6 to 8 hours can help.

Loss of appetite and weight loss

You might not feel like eating and may lose weight. Eating several small meals and snacks throughout the day can be easier to manage. You can talk to a dietitian if you are concerned about your appetite or weight loss. 


It is important to tell your doctor or nurse if you have a cough. This could be due to an infection, such as pneumonia. Or it could be caused by changes to the lung tissue, making it less flexible.

Shortness of breath

You might get short of breath while having this drug. Contact your advice line or healthcare team if you do.

Occasional side effects

These side effects happen in between 1 and 10 out of every 100 people (between 1 and 10%). You might have one or more of them. They include:

  • swollen lymph glands

  • swelling of the hands and legs

  • changes to sensation of touch either increased or decreased

  • tremor or shaking hands

  • redness (flushing) of the skin

  • redness, inflammation and pain in your eyes, much less common is bleeding in the eye

  • changes to taste

  • earache and hearing changes such as ringing in your ears (tinnitus)

  • heart problems such as palpitations, a high heart rate (tachycardia)

  • sore mouth and ulcers

  • skin problems such as a rash and being more sensitive to sunlight

  • night sweats

  • not able to get an erection (impotence)

  • nightmares

  • feeling drowsy

  • hot flushes

  • breathlessness and looking pale due to a drop in red blood cells (anaemia)

  • bruising, bleeding, small red spots (petechia), nose bleeds or bleeding gums due to a drop in platelets

  • changes to the level of hormones the thyroid makes. If low this can cause tiredness, feeling cold, weight gain, feeling sad or depressed. If high this can cause weight gain, fast heartbeat and anxiety.

  • infection

  • eye problems such as blurred vision

Rare side effects

This side effects happens in fewer than 1 in 100 people (fewer than 1%). You might have one or more of them. They include:

  • areas of swelling in the organs of the body symptoms depends on where it is in the body

  • hearing loss

  • inflammation of the thyroid

  • a low level of fluid in the body (dehydration)

  • diabetes

  • suicidal thoughts and hearing voices (hallucinations)

  • bleeding in the mouth, food pipe, stomach and bowels. Contact your healthcare team if you have an abnormal bleeding.

  • high blood pressure

  • wheezing

  • changes to how your liver works and liver cancer

  • pins and needles in your arms and legs

Coping with side effects

We have more information about side effects and tips on how to cope with them.

What else do you need to know?

Other medicines, foods and drinks

Cancer drugs can interact with medicines, herbal products, and some food and drinks. We are unable to list all the possible interactions that may happen. An example is grapefruit or grapefruit juice which can increase the side effects of certain drugs.

Tell your healthcare team about any medicines you are taking. This includes vitamins, herbal supplements and over the counter remedies. Also let them know about any other medical conditions or allergies you may have.

Loss of fertility

It is not known whether this treatment affects fertility Open a glossary item in people. Talk to your doctor before starting treatment if you think you may want to have a baby in the future.

Pregnancy and contraception

It is unknown whether treatment may or may not harm a baby developing in the womb. It is important not to become pregnant or father a child while you are having treatment. Let your team know straight away if you or your partner falls pregnant while having treatment.

Talk to your doctor or nurse about effective contraception you can use during treatment. Ask how long you should use it before starting treatment and after treatment has finished.


It is not known whether this drug comes through into the breast milk. Doctors usually advise that you don’t breastfeed during this treatment.

Treatment for other conditions

Always tell other doctors, nurses, pharmacists or dentists that you’re having this treatment if you need treatment for anything else, including teeth problems.


Don’t have immunisations with live vaccines while you’re having treatment and for up to 12 months afterwards. The length of time depends on the treatment you are having. Ask your doctor or pharmacist how long you should avoid live vaccinations.

In the UK, live vaccines include rubella, mumps, measles, BCG, yellow fever and one of the shingles vaccines called Zostavax.

You can have:

  • other vaccines, but they might not give you as much protection as usual
  • the flu vaccine (as an injection)
  • the coronavirus (COVID-19) vaccine - talk to your doctor or pharmacist about the best time to have it in relation to your cancer treatment

Members of your household who are aged 5 years or over are also able to have the COVID-19 vaccine. This is to help lower your risk of getting COVID-19 while having cancer treatment and until your immune system Open a glossary item recovers from treatment.

Contact with others who have had immunisations - You can be in contact with other people who have had live vaccines as injections. Avoid close contact with people who have recently had live vaccines taken by mouth (oral vaccines) such as the oral typhoid vaccine. Sometimes people who have had the live shingles vaccine can get a shingles type rash. If this happens they should keep the area covered.

If your immune system is severely weakened, you should avoid contact with children who have had the flu vaccine as a nasal spray as this is a live vaccine. This is for 2 weeks following their vaccination.

Babies have the live rotavirus vaccine. The virus is in the baby’s poo for about 2 weeks and could make you ill if your immunity is low. Get someone else to change their nappies during this time if you can. If this isn't possible, wash your hands well after changing their nappy.

More information about this treatment

For further information about this treatment and possible side effects go to the electronic Medicines Compendium (eMC) website. You can find the patient information leaflet on this website.

You can report any side effect you have to the Medicines Health and Regulatory Authority (MHRA) as part of their Yellow Card Scheme.

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